: 1) What is the most significant concern at highway speeds, with crosswinds
: while on long bridges with semi's - lift, or lateral movement of the kayak
: across the roof, or rotation of the kayak in a horizontal plane?

lift. We make Rail-N-Rack, (link below) and we have had both problems. On a pickup there is plenty of room underneath for air to get under and create lift. Tie your boat down to the bed good and you shouldn't have any problems with lift.

: 2) Does the shape of the kayak provide lift, or is it neutral? I remember a
: talk about the AWACS and the radar dome on the plane supposedly provides
: 10 degrees of lift ... I can imagine my kayak lifting out of the pick-up,
: rack and all!

It can happen you loose rack and all if your not bolted to the pickup. The kayak acts like a big sail with the wind coming up underneath.

: I've already blown my budget on the kayak building supplies, wetsuit, paddle,
: etc. and haven't even got the foot rails, PFD, etc. yet. Do I need to be
: concerned about a rack, too?

Yes definitely. Wouldn't you hate to loose your kayak if it blew off your pickup? minimum I would make sure to tie down to something bolted down. try Cal/grip tiedowns. If your rack goes in the stake pocket you need somewhere to tiedown on the bed. Cal/grips bolt on and would take care of this problem http://seaportmachine.com/calgrip/calgrip1.htm