Omar Khan wrote a post for Visual Web Developer Team blog which describes a workaround but doesn't explain why this happened in the first place. One big problem with that workaround is that you can't download it due to a broken link.
I found that one way to solve the problem is to copy mobile item templates from Visual Studio 2005 (assuming you still have it installed) to a special folder where VS 2008 will look for user item templates. Here's the detailed how-to:
1) Find item templates in VS 2005 folder:
2) Find user item templates folder:
3) Copy MobileWebForm.zip, MobileWebUserControl.zip, and MobileWebConfig.zip to that folder
4) Restart VS 2008. Mobile web items now appear in the "Add New Item" dialog under "My Templates":
One problem still remains: VS 2008 designer doesn't display mobile forms and controls. This isn't a major issue for me because I hardly ever use the designer.