Sometimes I’m called upon to set up a Google business apps account and migrate folks emails over. What can happen, depending upon how you do this is that you get thousand of emails in the inbox that are unread. Needless to say, the client doesn’t really have that many new emails. You can’t go in one by one and mark them as read. I just found a nice little post that shows you how to create a filter in gmail to handle all your email status’ at once!

1. Go to your Settings/Filters page and create a new filter.
2. In the Has the words field enter “is:unread” (without quotes), and click the Next Step button.
3. You’ll get a message warning you that this type of filter won’t be applied to new mail, but that’s OK. Click OK to continue.
4. Check the boxes next to Mark as read and Also apply filter to … conversations below.
5. Click Create Filter button, and you’re done. You might want to should delete the filter once you finish since it won’t be needing it anymore.

Here’s the original source:
http://www.apejet.org/aaron/blog/2008/06/08/gmail-tip-mark-all-unread-mail-as-read/