@Deidara: Checking for an update every time you connect is great, but the actual autoupdating is inconvenient. Also, most hacks now can simply be put in a purified pk3 and so an update wouldn't overwrite it. Plus, Tremulous has been forked and modded all to hell, with everyone using a different client; all of them would need to enable forcible autoupdating for this to work, and most users would loathe anything that is imposed upon them.
@rotacak: Same deal, there's a number of clients out there and each and every one (including the untouched 1.1.0 client) would have to be patched. Also, since it's open-source, the real cheaters compile their own stuff and won't be susceptible to this. Therefore, only the small middle ground would be covered: the nub cheaters who use the stock client wouldn't have the tattletale patch, and neither would the more leet hackers. Not a very well-thought out idea to begin with...

Amanieu and David are correct. You can't stop all cheating as long as you put any amount of control in the client, and to do any less is impractical. The goal is simply to minimize cheating, and while there are a couple of decent approaches to this, the best defense is a solid admin team on the servers you play on.