I'm running Vista SP1 with TV Pack 2008 on a dedicated HTPC, and using S3 standby instead of shutdown to ensure a fast wakeup when I hit the power button on the remote.

90% of the time this works fine.  But once in a while the fans power up but the system immediately freezes with a blank screen, and I need to power cycle it to recover.  The hardware includes an ASUS P5E mobo, an ATI HD2600XT graphics card, and a Terratec dual tuner.

I've tried upgrading the BIOS to v10 and upgrading the graphics drivers to 9.3 but the problem persists.  The WAF is terrible if the failed wakeup was intended to record a scheduled program.

Has anyone seen this problem before, or have any idea how I can diagnose the cause of the problem?

I had the same problem before. Turned out to be incompatibility between Hauppauge 1250 tuner and ATI3450 video card. They just did not want to work together. Had to replace the tuner card for another brand. Now sleeps/wakes up with no problems.

If you are using DVI or HDMI to connect your PC to your TV, make sure the TV is turned on WAY before waking the PC.  This will solve it as well:
http://www.gefen.com/kvm/dproduct.jsp?prod_id=8005

Again, only applies to DVI or HDMI.

Thanks for the suggestions.  I am using a DVI connection, so I'll try to reproduce the wakeup failure when the TV is powered on in advance.  If this works I need to check out the Geffen EDID device as I need reliable S3 wakeups for unattended scheduled recordings with the TV powered down.

If I still see the problem when the TV is powered on in advance, then I'll look into switching the tuner card.  I planned to do this when DVB-T2 HD tuners are available in the UK anyway.

Either way, I'll post the results - and thanks again for the suggestions.

Unfortunately it turns out my S3 wakeup problem is not EDID related, as it still happens when the TV is powered on well in advance of the PC wake up command.

I'm guessing it's some kind of hardware/driver/bios S3 incompatibility which means my options are reduced to swapping hardware and looking for a working configuration by  trial and error. 

That's definitely not EDID. With EDID system doesn't freeze and you just get black screen which is easy to fix with Alt+Enter. You most likely have hardware/drivers incompatibility. I had the same problem for a month (had to reboot after wake up) until I found what was causing this. I'm still not sure if it was Hauppage Hvr-1250 or EAH3450 graphics card or my MOBO. But they just couldn't work together.