The key in filter media is how fine it is, and how much surface area it offers. Most of the humidifier filters I've seen are rather course, and thus would fare better as mechanical filters (which catch the big stuff like plant material) rather than biological filters - so if you are using it with a finer material, it could be ok. Also, most humidifier filters have been treatd with an anti-microbial agent to deter mildew and mold, which would most likely hamper the growth of bacteria. It probably wouldn't hurt the fish, but it could really mess up your cycle. That's my opinion, anyway.
Realistically, though, humidifier filters are usually much more expensive than replacement cartridges for aquarium filters.