I recently purchased a Canon 50d and I am curious about the extent of the weather sealing. When reading, I noticed that seals around the battery compartment and the memory card slot were mentioned. What level of protection against dust and moisture does this afford me?
The main issue with any "how weather sealed is my Canon DSLR" question, is that they are not rated to a depth of waterproofing. And how else would one measure this? "The Canon 50D can withstand the rain of 1/4" per hour coming perfectly vertical with XYZ lens"?
Canon does not provide this information, and the information you will find on the internet will vary depending on each personal experience. As Itai noted, it does have some sealing, which is in effect some foam material and some rubber seals. To get better sealing add a "L" series Canon lens with weather sealing, and potentially a filter to seal the front of the lens if the lens needs that. Beyond that information, it is pure conjecture how good the sealing will be in the specific environment you shoot in.
Do you need weather sealing? Then check out Pentax or higher end Canon DSLRs. Is it raining/snowing? Protect your camera unless you don't mind the potential for failure.