Evolution 3.0.3 on Fedora 15 x86_64.  I'm connecting to an Exchange 2010 mail server with evolution-mapi 3.0.3-2, but I don't think that's relevant.

Assumption: you have a subfolder under your inbox that contains some mail.

1) search the inbox for an arbitrary term (this works correctly)
2) switch the view to the subfolder

Observe that the search bar returns to the "Subject or Addresses contain" message, indicating a cleared state, yet the term you searched for in the inbox is still in effect.  I doubt this is the intended behavior, since the contents shown for the subfolder are filtered based on the original term, but the search bar doesn't give any indication this is happening, nor does it permit you to clear the search term (you have to go back to the inbox and clear it there to get rid of it).

3) search the subfolder for a different arbitrary term

Observe that this does what you expect, filtering the subfolder by that term

4) go back to the inbox

Observe that the inbox is now filtered by your term from step 3, but the search bar indicates it is filtering by the one from step 1.  This is clearly wrong.