Caveat: I'm new to selenium so maybe this feature already exists.
A common test is to test the HTTP status code (return code) of a web page.
There does not seem to be a way to do this in selenium / selenium RC.
Yet, it's a fundamental feature of web testing.
A short thread on this: http://clearspace.seleniumhq.org/message/32319
A related feature request: http://jira.seleniumhq.org/browse/SRC-409