This is a great point. It might seem extreme, but I would advocate never using the User-Agent string to make decisions about what to serve a client. There is too much hackery and history that clouds up the User-Agent (such as every browser identifying itself as Mozilla), and it's almost always a proxy for something else that you actually want to test for.

In some rare situations, it's unavoidable, but even then I'd urge trying to rearchitect the solution to avoid it.