Steps:

1. Open Banshee
2. Click on the search field to focus it
3. Type some characters that return no results
4. Press ctrl+a to select everything in the search field
5. Noticing that nothing happens, ensure that the search field is focused and try again
6. Still no luck

This seemed like a straightforward bug until I tried:

1. Open Banshee
2. Click on the search field to focus it
3. Type some characters that return results
4. Press ctrl-a, expecting to select the text in the search field, which has focus. Instead all of the tracks that match the search are selected.
5. Press ctrl-a again, just for kicks.  At this point, the text in the search field is finally selected.

I'm guessing this was intentional, to make it easier to select all tracks after performing a search, but in my opinion, this is not very intuitive.  I expect the shortcut to apply to the field with focus.  The problem is even worse when the search returns no results.  It seems logical at that point to select your text so you can try another search, but the shortcut doesn't work at all in that scenario.