Dropping support for older versions of Windows would be consistent with Microsoft's past behavior and efforts to move Windows to a single code base. The consumer and business versions of Windows XP, for example, share the same lineage as 2000 and NT, which are considered to be 32-bit operating systems. In the past, Microsoft supported two separate code bases, one for businesses based on Windows NT and another for businesses and consumers based on Windows 95.Since NT 4.0's official support period is coming to an end, it's no surprise that Windows 2000 is the oldest 32-bit OS that Microsoft intends to support with Office 11.
No doubt critics will hammer Microsoft for trying to force users of older operating systems to upgrade, and they'll have a point. Still, I'm not convinced this is going to be a big deal for many. Few users do much more with Office that can't be handled by older versions like Office '97, so I'm not sure many will be clamoring to upgrade to Office 11, especially if they've been neglecting their OS and are still running something like Windows 98. In fact, I'll bet that most users' office productivity software needs would be satisfied by OpenOffice.
Of course, we should keep in mind that Office 11 is still in beta, so its OS compatibility could change between now and its final release. Microsoft may hold back if they find this decision alienates enough of their target market, who may be tempted to migrate to one of Office's competitors.