I have been using jQuery IntelliSense in VS2008 and it has been great. Recently I added a reference to jQuery UI and since then, the jQuery IntelliSense has went away. I found
It's likely that there's a bug in one of the subsiquiently referenced JavaScript files. Open your JS file and once the "Updaing JavaScript Intellisense" has gone from the status bar of Visual Studio (there is a menu option which will force the JS intellisense to refresh, don't remember where it is, I just created a keyboard shortcut via the Tools -> Options -> Keyboard area) open up your Errors window and under the Warnings you should find the reason why the intellisense has failed to load.
It's generally a bug found when parsing one of the files but I have had stack overflows when I had a lot of files referenced.
Edit: You also should make sure you have this VS patch installed: http://code.msdn.microsoft.com/KB958502 and VS 2008 SP1 (install SP1 first!). Then you just need to have:
///
Ensure that you maintain the -vsdocs
on the intellisense file and it will be automatically picked up (as long as it's in the same folder as the file you reference)