I did not find any post or bug entry related to this problem, so it's probably just me...
However, nothing is being updated from the server. As a work-around, I have to right click on the solution and the click "Version Control" / "Update". I'm using subversion.
The last known Xamarin Studio version where it worked was 4.0.10, the first known version where it was broken was 4.0.13 (still broken in 4.2.2), so the bug must have been introduce in 4.0.11, 4.0.12 or 4.0.13.
Anyone else having this problem?