Using the following hardware:
NF3 motherboard
GeForce 6600
Hauppauge PVR 250
Gateway VX700 monitor

I updated the drivers using NVidia's setup file using this wiki page (the Current Way).

Settings are Composite 0 and video0

Looked through all log files that BlankScreenWhenWatchingTV told me to. I saw no errors in 250, and my video card is working fine.

My card definitely works in Windows XP with Hauppauges's software.

My zap2it is configured properly, it shows the correct shows' titles when flipping channels but no TV!

Good work so far.

Did you also try the other frequency sets?

Did you try the ivtv driver upgrade? It takes all of about 5 minutes, the directions are VERY detailed, but you just have to cut and paste. Posting your log extracts would also help. We might see something that wouldn't be obvious to you.

Did you triple check you input connections? Both in the mythtv-setup and physically? What are they? Was composite your input (from a cable box?) or you output (to the TV?).

If you're using a cable box is it on and tuned to a valid channel? Have you checked the output with a TV?

If you're showing correct titles then your program guide is probably configured correctly.

OBTW - Your new directions aren't quite complete. If you didn't disable the auto install of the nvidia drivers it will revert to 1.0-7174 when you reboot. Also if you have to put back a saved XF86Config-4 you need to do a gdm restart.