Never, never check it.

I don't carry mine when on business travel, so I only have to worry on family vacations and the like. On those trips we have carry-on allowances for the 2 kids so up to 4 bags/4 personal items covers us. I have both a Canon shoulder bag and a Minitrekker backpack that up until now have managed to contain my 10D/grip, 4 lenses, filters + misc.

But I have a 100-400L IS arriving the day before our Hawaii departure next week and I have no idea what I'm going to do. I does come with it's own case, and I suppose I can fit my laptop/charger in the minitrekker just for the trip... And I wonder if I need both the tri- and monopod...

If you do want to carry your gear frequently, you'll probably have to get used to checking the duffel/suitcase on every trip, unless you can squeeze it into the rollaboard. Unless it's a long trip or you have specific needs, you can usually get away without extra batteries, filters, lens cleaning supplies, etc., and take just one lens. Pack smart with the rollaboard, using all the outside pockets, and it might be possible.

Someone above made a good suggestion - check dpreview.com for literally hundreds of folks who have been in the same position.

(then again, if you're the compulsive type and don't want to end up with four L lenses, including a 100-400 4 lb. monster, the latest/greatest Photoshop CS2 and the requisite additional RAM, cleaning devices for that "oh-so-spotty-under-test-conditions" sensor, etc., etc., maybe you should stay away from dpreview... )