Missing intellisense in skin files is very annoying, but here a simple trick to fix that problem!

1. Go to Tools->Options menu.
2. Pick Text Editor -> File Extesion fom a tree at the left part of Options dialog.
3. Type skin in Extesion text box.
4. Select User Control Editor from Editor dropdown.
5. Click Add and then Ok to close dialog and re-open your skin files.

http://vladimir.bychkov.info/blog/PermaLink,guid,be76b3bf-c524-456c-9c8f-a2584924aa32.aspx