Screw the TSA and screw the airlines who go along with them and screw the EU for bending over to the US and adding the stupid baggie rules. I drive in the US and take the trains in Europe and stick pretty much to intercontinental flights. Except of course when I am in Asia where they somehow have retained a bit of common sense.

What's the speed limit from LAS to LAX these days?

The previous posters all mentioned that the decision to drive versus fly is largely a matter of personal preference. For me, I'm a) lazy and b) love a bargain. I view travel in dollars and minutes, not miles. For me this means it usually works out to fly medium distances (i.e. >200 mi). I live in San Francisco and love to visit Tahoe. My calculus for winter 2006-2007:

Drive
I work near the Caltrain Station and it takes me ~30 minutes to bike home
My car gets ~28 mpg on the highway (I drive a 2 seater with AWD)
The distance is ~208 miles .
Gallons of Gas used: ~8 (*2 for round-trip)
Current Price Per Gallon: $3
Marginal Cost: $48
Time is >8 hours assuming no stops

Fly
BART from SF Embarcadero Center to OAK cost $5.35
Dep: 6:32p Arr: 7:15p
OAK-RNO WN2383 8:10p-9:00p $118.60 inc tax
Pick-up old Car at Reno Airport, drive to Squaw 45 minutes
(distance 45 miles, 10 mpg gas $2.25 per gallon)....cost $35.26
RNO-OAK WN306 9:05p-10:00p
BART/Muni from OAK to Home cost $6.60
Dep: 10:30p Arr 11:53p
Total Cost RT=$165.81
BUT...a WN free ticket is worth ~$300 on eBay
and I earn 1/8 of a free ticket with each round-trip (i.e. $37.50)
So...the all-in cost of flying is $128.31, or $80 more than driving.
And time is a consistent 8 hours...

This all means that, assuming that a) gas prices stay the same and b) WN does not lower the effective price via either a fare sale or RR bonus...it makes more sense to drive.

It's 70. You can comfortably drive about 80 without attracting attention, assuming the traffic isn't schizoid.

I've tried all of the above options before and I can vouch this. I'd dare to say that, there's a bunch of bugs flying all over around US-95. So, your vehicle's radiator/oil cooler could get damaged. I had to wipe them down every gas station to prevent from overheating.
As for I-395, you'd better bring tire chains with you. Last time, I encountered snow there and was offered a lovely $98 for a set of chains alone at the gas station. 18inch, though.

The answer for me is about 4.5 hrs for business and up to 8 hrs for personal travel (depending on the fare). :p

I think the OP summed up well why short hauls have taken a real hit with business travelers with the post-9/11 security and all. Not only is the time situation upset with longer delays at the airport and the possibility of a very long security line, but the travel-related TAX situation on the money side gets worse. Car rents are typically loaded with taxes, sometimes for unrelated things, and airfares also take hits for PFCs, the segment tax, the security tax and the ticket tax. All that "overhead" is avoided with driving.

That said, I'm an almost 100% leisure traveler, and flying has helped turn me off of drive trips for any real distance. Seems every time I get on a long-distance interstate out of ATL, even to go to BHM or something modest like that, it's an unpleasant experience with too many cars/trucks trying to do 80 on an interstate that doesn't have enough room in many places to do that and to keep a safe following distance. It's easy to see why the roads are many times more deadly than the skies, and why 1,000 or so extra people died on the highways vs. the usual in the 3 months after 9/11.

One thing the tax situation does, though, is make me favor fewer longer-haul trips over more-frequent shorter hauls.

My formula is, if you have to ask, drive. You will know when the only way to get there is to fly. But flying doesn't save any time if it's less than 6 hours and even less than 15 hours, it's pretty shaky financially when you consider that by driving you have your own car and not some rental. For instance, my husband heard the news reports of the liquid ban and put the kibosh on my plan to use free miles to fly to Midland/Odessa from New Orleans and then rent a car to get around Marathon/Big Bend. Even considering we would have flown free, it turned out we saved money -- he purchased several heavy items on the trip that would have cost significant bucks to ship motor freight. They wouldn't have flown at all due to the weight. You wouldn't even think of such things when flying. You limit yourself without even thinking about it.

One time I got the dumb idea to fly to my folks in Knoxville from New Orleans -- a 9.5 hour drive. Flying over there was OK. Flying home, there were tornados in Memphis or some such happy horse puckey, and I had to fly all over creation to finally get back to New Orleans. If ANYTHING goes wrong, you save no time by flying, yet you have to rent somebody else's stupid unfamiliar auto.

When I lived in Seattle I would drive to Whistler in the north, Spokane in the east, and down to Bend/Eugene in the south. I have done the drive to Sacramento and Las Vegas before. But I don't think I would again.

Now living in Lugano, CH I train to places like Milan or Rome or Zurich but rent a car to go to Bern or Munich. I fly to places like Amsterdam, Spain, or Morocco. I think alot of it depends on how soon I need to be there (or be back), how long my vacation is, how much it costs to drive vs. train vs. plane.

2.5-3 hours is my cutoff. Philly is a drive, Providence is a drive, Boston and Washington are flights.