Last weekend the patch for bug #437483 got committed (it looks like it went to CVS head and branch only, not to mozilla-central -- no idea how that works now). This means that the install.rdf now contains a line like
The extension manager uses this information to determine if the addon will work on your platform. Unfortunately, this broke our build scripts and the nightly XPI refused to install.
A similar issue came up with the Firefox 3 release build, which didn't work with Firefox 3.0.1 any more.
Both issues are fixed. Please re-download the XForms XPI and you should be ok.