Can players on a PC and an Xbox play together in any way? I.e. can Xbox users connect to a (modfied?) PC server, or vice versa?
The two versions are not compatible and can not be played together, especially to a modded version of PC Minecraft.
As mentioned by @MBraedley, the version that was ported is an older version of minecraft (though what I have read says it is based off of 1.6, not 1.8 beta). The matching for minecraft-360 uses Microsofts XBL systems and such, which PCs do not have access to. And a further incompatibility is that minecraft-360 is limited to a world size of 1000x1000.
It's not compatible for now. Don't forget that it has just been released and there are several updates planned by Notch and his team. I heard that Notch wanted to grant the compatibility between Xbox Live Arcade and PC games, though he doesn't know if it can be made this way. As we often say: Wait and see.
Though modding and designing your character is (will be ?) an integral part of the xbox version. (Since you can't modify the game folder on your xbox as you do on your computer)
Machines that host minecraft as one of its games can not connect with other machines (With an exception of Ipads, Ipods, and Iphones; if one counts them as seperate machines, that is).