The problem we've seen visiting different caches is that the zip-n-seal type bags used to keep things clean and dry quickly wear out. After being opened and sealed a lot, it can be really difficult to get them to seal properly.
These look like a great alternative: Clip-n-Seal. Apparently they can be cut down to size, so they'd be easy to fit into a cache. Plus I can't see how they'd ever really wear out. Looks like they're designed primarily for sealing food, but no reason why they wouldn't work for any number of other uses.
I think we'll give them a shot in the next cache we hide and see how it goes.
Just thought I'd share!
drew.