We've discovered one issue caused by VS SP1 installation recently. After having upgraded to SP1, IronPython disappeared from language dropdown when trying to create a new web project.
The issue is due to a registry key for IronPython under NewProjectTemplate was removed by VS SP setup. Adding it back fixes the problem. To do that, the easiest way is to uninstall and then reinstall "IronPython for ASP.NET CTP".
This happens to Visual Web Developer SP1 as well. Similar work-around applies.
Note, you don't have to do anything if your IronPython web project still shows up after upgrading.