I looked all yesterday, and couldn't find a way. I was trying to do it with the Email app, since I was setting it up to check my work email when I was going to be out of town and I get so many emails that I don't need to deal with until I get back. I only use Gmail for personal stuff, and I have no problem dealing with that. I ended up downloading K-9 seems to work fine, a little confusing in parts, but it does have a "mark all as read" feature. The only thing I don't like it is difficult to determine that which are read and which aren't, there is just a very slight variation in color intensity.