According to what everyone else seems to bring with them when shooting I might overpack lol. I would just hate to get to a shoot and have it at home and not with me and need it. It's the old boyscout in me "be prepared" that hates to leave anything at home.

Camera & Lenses

1Ds Mark III
85L f1.2 II
24-105L f4 IS
70-200L f2.8 (non IS)
16-35L f2.8 II

Flash & Accessories

580EX IIX 2
580EX
ST-E2 Speedlight Transmitter
25mm Extension Tube II
TC-80NS Timer Remote Control
Manfroto Carbon Fiber Tripod & Ballhead
Various UV Filters, Circular Polorizers, ND

Everything nicely fits in my Lowepro Pro Trekker II AW Camera Backpack.

When I need to travel light for a hike i have a smaller Lowepro backpack that I usually take my 16-35, 24-105, 70-200, 580 ex II, tripod, cable release & filters.