Most weather protection mods completely seal of the body and lens of the camera using Nylon (or some sort of nylon, at least). A very common hack is to use a UV filter as a front element for the lens and cover the rest with a bag.
The problem there is that drop on the lens cover may interfere with the shot. The folks at Digital Camera World came up with a different solution that uses a CD spindle for lens protection.
While this solution is not 100% tight, it looks like for non windy weather it provides good protection while removing the risk of drops on the lens of filter. (Yes, it also uses a nylon bag, I guess there is no way around this one).
If you want a more refined solution, you can buy similar “dedicated” bags for about $6-$8 in B&H.
The only question now (2013) is where the heck does one find an empty CD case spindle?
[why a blank CD case is the perfect rain guard for your lens | Digital Camera World]