Currently, the plans for IntelliSense are to support SQL Server 2008 *ONLY*… since it works against SQL Server 2005 in the February CTP, I was very surprised to learn that it is being dropped by RTM (and possibly by RC0). This is mainly because of the time frame of the release and the difficulty of catering to multiple dialects (sometimes there are going to be false positives, and there are even possibilities that it will miss obvious issues). You can read more about this issue at The W Blog.
Being that IntelliSense isn't even going to be complete when SQL Server 2008 ships (mostly just supporting SELECT operations), I could live with a disclaimer that says, when you are working against a downlevel version, it is not always going to be 100% accurate. Could you? If so, I urge you to cast your vote on Connect (at publish time, only 11 people have voted):