Don't high speed modems transmit and receive on the same frequencies, using echo cancelation to decode the receive signals? Does that make it impossible to eavesdrop on high-speed (i.e. V32bis) modems?

No, and a lot of crackers and phone phreaks found out the hard way. You can buy protocol analysers off-the-shelf that will give a dump of the entire communication by just passively listening in (or possibly playing back a recording). I have seen units that could decode all of the popular Blue Book protocols for consumer equipment such as faxes and high-speed modems as well as ISDN, T1, DS3, ATM, etc... Most are programmable and some are full-blown computers running stripped down versions of Unix and can also be controlled over the network from RealComputers. With multiple analysers and a little custom software you could easily perform MITM attacks. The hardest part is getting in the middle. Modulation, comm-protocols, and compression techniques are not a replacement for honest to goodness crypto. andrew