eWeek's Ben Charny discusses how Regulations Threaten Nomadic VOIP in the Nov. 29 issue. This is a really interesting discussion and one which directly threatens services such as Skype. In effect, regulators are trying to force VoIP into the TDM world, which means that the benefits of VoIP (e.g. the ability to use your phone from anywhere) is lost.
There are solutions, such as requiring a user to confirm their location each time they make a call. However my understanding of emergency services databases is that they can't update user location information in real-time, so even if you submit your address, it won't be available to 911 operators for hours or days.
(thanks to Jeff Pulver's blog for the pointer)