aha.. peetza gets a cookie, or pizza, or something. Searching for nonexistent words is what reproduced it for me.
BR: Is your IE8 symptom fixed?
There were two lines at the top that I added in when we switched to the sphinx search engine, to show the number of hits for each word in the search string. (I admit it wasn't kosher to output stuff before the html.) Sadly, it appears that vbulletin doesn't enable zlib compression until after those two lines are printed. The http headers tell the browser that the response is compressed, but those two lines weren't, so it threw a fit. They might not have been getting counted in the content-length header, either.
What I don't know is why it was working before. There must have been some behavior change in a recent version of php or lighttpd. It might have been output buffering. Maybe php used to wait until the output buffer was full before it checked whether to compress, and maybe now it compresses on each and every write to the buffer. Seems inefficient, but I have no idea, just guessing.
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.