Someone seems to have hacked into my MacBook Pro.

This sort of thing is virtually impossible to do unless you (or someone else with physical access to the machine) have set up some kind of remote screen sharing. If you're on a wireless router, that makes it doubly difficult... the wireless router acts as a kind of barrier for incoming connections. Nobody can target your machine unless they're on the same wireless network or there's software on the machine (like LogMeIn) that allows the remote connection.

So, make sure your wireless network has a good password, make sure everything in System Preferences -> Sharing is either off or protected with a strong password, and make sure you don't have software like LogMeIn installed. If you've got screen sharing turned on for a good reason, and it's protected with a good password, but you also have MobileMe, change that password. If anyone untrusted has had unmonitored access to the machine, you need to go over it with a fine-toothed comb and remove any software that didn't come with the machine and you didn't install.

The one exception to everything I said above is malware. However, malware that installs backdoors is pretty rare on the Mac. (See my Mac Virus Guide.*) Even then, those backdoors are things that can be used through automation, not for screen sharing. Malware creators don't have any interest in hacking one machine at a time, they're writing malware so they can automate hacking of many machines. I won't say this is impossible, but it's extremely unlikely.

* Disclaimer: links to my pages may give me compensation, and should not be taken as endorsement of my services by Apple.

I would switch away from WEP as soon as you can. The proper software can crack WEP in just seconds. WPA/2 is a more secure and reliable option.

I realize it might be virtually impossible for someone to hack into my machine, but I know what I saw.

I wasn't implying that you were incorrect about what you saw, I was giving you a way to focus your energies... it's virtually impossible "unless you (or someone else with physical access to the machine) have set up some kind of remote screen sharing." You have to figure out how that happened. If all sharing services were off at the time, that leaves third-party software installed either by you or someone else. Some companies offer this as a "service" for tech support, which I don't trust... have you given such a company access? Has anyone untrusted had access to the machine? You say you confirmed it wasn't someone in your house, but are you sure of that?

I've seen people steadfastly claim to have seen this sort of thing with everything off in sharing, and who were absolutely, 100% certain that nobody in their house was doing it, absolutely, 100% certain that nobody untrusted had access to the machine and absolutely, 100% certain that they hadn't given access to someone. The problem is, those folks are guaranteed to be absolutely, 100% wrong about one of those things. Don't get yourself hung up on this because you're making assumptions that may not be correct.

If you can't be sure that someone hasn't gotten to your Mac and installed some software to give themselves a back door, you should probably get a copy of Little Snitch. That will prevent applications on your machine from sending data out over the network unless you approve it. You'll have to learn a whole bunch about which applications are normal parts of your Mac's system and make connections all the time, but in the process you should shake out any possible culprit. Just make sure you don't approve any connection attempt until you verify that it is to be expected.