You can add a bookmark to Safari's Bookmarks Bar that will display the number of unread Gmail messages you have.
First, go to the link feed:https://mail.google.com/mail/feed/atom.
Then, bookmark it in Safari. I called it ':' so that it wouldn't take up much space, and put it right after my Gmail bookmark so the result is: 'Gmail :(1)'.
In the RSS tab of Safari Preferences, set the Check for update: menu to 30 Minutes so that you have current results.
Now, send yourself an email.
Once the feed refreshes (you can quit and reopen Safari to do this manually), you should see the number of unread messages in your inbox in parentheses after your bookmark.
[crarko adds: I tested this, and it works as described. I had to play with the URL a bit to get it to work right; in the original hint it was given as feed:https://mail.google.com/mail/feed/atom/inbox/. That didn't seem to work for me so I just went to the Gmail inbox in Safari and switched into RSS mode and grabbed the link now listed in the hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101220111704264