I didn't want to start a whole new thread, but I have the same problem at http://www.runningisfunny.com and I notice a lot of other people have it. Deleting the theme and plug-ins just led to internal server errors or a blank page. The log-in php link (under the "Meta" menu) has been deleted, taking you right back to the home page if you click it. Going directly to the log-in page just leads you to a non-functional text version.
I removed all files through FTP and reinstalled a clean version of WordPress, but despite careful work I still got the internal server error. When I restored the theme folder (I use HeatMap) my site at least returned to the state you currently see.
I'm no expert, but my database shows no obvious malicious code, and no users other than myself. Network Solutions will restore my database to its condition earlier in the week. Hopefully that will eliminate the problem, which just showed up this morning.
This is the first time I've had a problem that even a reinstall didn't fix, so I'm worried that it's a particularly insidious database hack or at least a very strange bug.
I'll be monitoring these boards, but if a solution is found I suggest giving it the widest dissemination possible. I think a lot more people are going to run into this one.