Any easy way to get rid of the Gmail spam folder count within Mail is to set up a filter in the Gmail web interface. In the "Has the words field," enter in:spam and "mark as read" as the action (if you don't have the "mark as read" option, try changing the language to English (US)). The spam folder count should then be gone.

You can refine the search further if you wish by adding words to the "Doesn't have" field, (e.g. your name, your work etc...). This will then highlight any messages that may have been marked as spam erroneously. Now if only someone can figure out a way to do the same with the All Mail folder!