One of the reasons Unreal Tournament 3 isn't showing up on Xbox 360 this Fall is Microsoft's approach to user generated content. At E3, Microsoft Game Studios head Shane Kim told 1UP that Microsoft wants complete control over what's transferred through Xbox Live, thus preventing Epic Games from allowing PC-created mods (a step further than just levels, as these could be completely new modes or even games) to appear in Unreal Tournament 3. Sony has no such restriction, thus helping its appearance on PlayStation 3.
At Gamefest 2007, however, Microsoft's official line was less clear. To deliver user generated content, developers can take advantage of what's called Xbox Live Silver Platform (XLSP), where the data transferred runs through the publisher's servers, not Microsoft's. As such publishers become responsible for what runs through it, monitoring and policing offensive content, etc. Taking advantage of it, however, requires Microsoft approval.