File under 'not surprising' but still 'disappointing':
It looks like one type of application you won't be seeing in the new Windows Phone7 Marketplace (besides adult themed ones) are ones that use the BitTorrent protocol.
In a back and forth I had with Blue Planet Apps, they revealed that they won't be bringing over their Android 'DTor' BitTorrent client to Windows Phone 7. Evidently, Blue Planet Apps had already spoken with Microsoft on the issue:
...[Microsoft] says that torrents represent an unknown legal issue which would violate their policy...
Legal or not, it sounds like Microsoft just does not want to touch the controversial file-sharing system with a 10-foot pole and would rather just avoid the whole issue (and thus maintaining their relationships with their software partners, natch). Here is a case-difference between WP7 and Android as theGoogle's market place does allow 'DTor' to be sold, no questions.
Update: BluePlanet clarifies in comments that Microsoft is not 100% against a BitTorrent app, but rather this is all new for them so they're taking it step by step on these grey area programs.
Fun factoid: Windows Mobile was the first mobile OS to have a BitTorrent client, Adisasta's WMTorrent, which is still available and is quite a nice program.