I have tried unsuccessfully to install Ubuntu for the first time on a brand-new ASUS P4P800 SE mobo with BIOS version 1010.007. It would not even mount the root file system, and I kept getting "The drive appears confused" error messages.

Luckily, I could solve the problem. This post is for all those out there who are running into the same problem with this mobo:

The ASUS P4P800 SE BIOS version 1010.007 appears to be incompatible with any debian 2.6 kernel, including Ubuntu, even though it runs fine with XP and even with the 2.4 debian kernel. Once I got the 1011.001 BIOS version off the ASUS web page and installed it as described in the ASUS manual, Ubuntu installed without any problem.

With that BIOS version, I'm running (K)Ubuntu now for almost two weeks without any problem, and I'm becoming a true Ubuntu fan.

Awesome of you to share your solution and welcome aboard to Ubuntu
By they way, could you change your title name to something like "with solution" or "(problem solved)", just in case someone searches. That way they'll find the answers faster hehe

Did you buy and install Asus' wireless add-on card for that motherboard, and if so, how would you rate its performance so far?

Sorry - I have no experience with that wireless add-on - never used it. Ubuntu recognized the built-in sound and ethernet though, and both are working fine.