Actually this has been discussed in length already (you can probably do a search and find it).

Interestingly, every article I've read mentions CO specifically as practicing the "bring as little fuel as possible" mantra. I haven't been on one of the diverted CO flights (cause most of my flying is on NW) but I have coworkers who have been on several of them. As soon as a plane can't land at it's destination (e.g. EWR or IAH), it will circle for up to 10 minutes and then divert. SEA-IAH was diverted to AUS. SEA-EWR was diverted to SWF or PHL.

As mentioned, most of my flying is on NW and I've had to circle a few times near MSP (in and of itself a rarity). We circled for up to 25 minutes with no problem and still made it to MSP. While I'm pretty sure almost all carriers are flying with less fuel to save weight and ultimately save fuel, it hasn't affected me directly yet.

I definitely expect more diversions, especially for planes going to EWR - still the most delayed airport in the country. With many airlines cutting service it remains to be seen how this will affect ORD, LGA, EWR, JFK and the other delay prone airports. Will they actually get back to a normal level of service? That alone will save a ton of fuel for the airlines, especially CO, UA, AA and DL.

-RM