This was essential because (for some reason unbeknownst to man) you can't drag a WinForms user control from the Solution Explorer onto a form, you have to do it from the toolbox (which sucks: the ASP.Net team managed it just fine, so what's the issue?).
In VS2005 the same functionality exists (with the additional feature that it can be disabled for performance reasons with the AutoToolboxPopulate option). But it doesn't work - or not for us at any rate.
Eventually it turns out (thanks Cam) that it doesn't work for projects within solution folders. How crazy is that?
"The fix probably won't make it into Service Pack 1, but I will forward your feedback to the appropriate team for consideration."However the bug reports says it's fixed already:
[Ben Bradley, MSFT, link above]
...so I'm crossing fingers for SP1 (whenever that comes out).
[Update: Didn't publish this for ages, but no, SP1 doesn't fix it. Poo]