Have you read Stephen King's
On Writing? It goes into depth on this very issue, as I recall (can't remember which chapter, but King points out that passive verbs are best used in things like non-fiction essays and technical manuals, where you want to retain a certain amount of emotional distance in the writing.) An example of a passive verb is "The knife was taken from the kitchen table", whereas "He took the knife from the kitchen table" uses an active verb and is thus a stronger sentence. Personally I don't have a problem with the occasional passive verb, but it's easy to go overboard with them, not unlike adverbs. I suppose it's something best dealt with in your rewriting process. Hope this helps - I'm recovering from a nasty bout of flu, and my brain is still pretty fuzzy.
-Liz