iPod Touch and e-mail viruses

Yes, it is safe.

There are no known exploits for iPod touches, and, even so, it certainly can't execute .exe files

Again, very doubtful at this moment. iTunes is picky about what it syncs anyway. I'm sure the AV companies would pick up something like this before you did, so I wouldn't worry

For example, could something get transferred to my MacBook next time I sync my iPod or something?

No. iTunes only syncs the music, videos, podcasts etc. that it supports. It doesn't sync virus-laden email attachments or anything else.

Secondly, the iPod touch does not mount on the desktop, so nothing can get transferred that way.

Thirdly, the iPod touch is not visible to other devices when on an a network, Windows or Mac.

If you download, install and use a specific App for sharing/transferring files (such as AirSharing), any transferred files can only be accessed by the App in question, and files can't magically copy themselves to the iPod touch, as nothing has access to the iPods file system.

Fourthly, all iPod touch applications are "sandboxed" meaning they only have access to their own data, not that of other applications or the operating system. So nothing can be transferred that way.

And finally, even if, hypothetically, an .exe file virus from one of your emails somehow managed to find its way onto your iPod touch or Mac, it wouldn't do anything anyway, as .exe files don't run on iPods & Macs.

So, in short - you have nothing to worry about.