Another option for waterproof panniers is to make your own out of square buckets with lids (ask for them at a local bakery, or use the ones kitty litter comes in). It's easy to find instructions online and they're easy to make with a few simple pieces of hardware. Much cheaper than buying fancy panniers. The only downside is that the lids can be a pain to get off.

Here's a link to the instructions I used to make a set a while back: http://www.crazyguyonabike.com/doc/index.html?doc_id=1841