Would it be wise to combine all CSS
into a single file? Would there be any
performance increase. I have to assume
that an HTTP request is made to get
each file on initial page load, and
reducing those requests would seem to
make sense.
Yes, but make the combination at build or runtime and don't try to maintain a single file if you started with multiple ones.
In addition to the number of HTTP requests it is also important to set the right expiration headers in the response.
Are there any reasons NOT to combine
all css into a single file?(such as
maintainability or other performance
issue)
It is not necessary to maintain a single file, but good to serve a single file, because CSS data is anyway merged.
The YUI Compressor is a good tool for JavaScript and CSS minification.