This happened to me yesterday, I upgraded to 3.0 about two weeks ago, and yesterday i couldn’t login to my admin page (or view my blog). My server was fine, and i could login to my webspace and see that all the files were there. I just had a blank page when testing in multiple browsers. (my main domain sites were fine, but my wordpress blog would not show at all).
The problem lasted about 10 hours, although i logged in this morning and it was fine (no problems).
I had a similar thing happen about a month ago. I contacted my webhosting providers and they said it was a problem with my ISP.
Anyway, the problem returned yesterday and i thought i’d leave it overnight just to see if things worked the following day. I regularly backup my blog, php and xml databases, although i didn’t want to start reinstalling my blog, so i just left it overnight and it is fine this morning.
If anyone experiences similar problems like this in the future (blank admin page/blank blog), i would recommend leaving the blog overnight, and checking the following day. I’m not sure what caused my blog to fail, but as mentioned earlier, i left it overnight and it is working fine today, and it saved me a lot of unnecessary work in rebuilding the blog.
Hopefully this will help others who experience similar problems (blank admin screen and blank blog page).
I realise that this may not solve identical problems for other users, but its worth a try.