Mark,
I just tested on the dev site with IE8. I'm assuming SEF is enabled (I didn't check), but logging in worked fine for me. After logging in, I always remained on the same page (which I'm assuming is how you have it configured).
So, the obligatory questions...

- Is there a specific page that you log in from and get the 404 error?
- What is the link in the address bar that's giving the 404 error?
- And, of course, does this happen from different FB accounts and/or on different computers?
We'll get ya going, don't worry!