Has silverlight replaced or killed off XBAPS?
I guess you are refering to the Silverlight 4 and its extended priviledges. Well it certainly eats into some area where you would have had to use an XBAP before. However XBAPs still have a wider access to the client system and the full range of the standard .NET framework.
I agree with Anthony. The number of situations in which you would use an XBAP has gone down pretty significantly (not that there were that many to begin with). And indeed, for most LOB applications, I'd strongly consider Silverlight 4 ahead of WPF, and only go with WPF if you really needed something that you knew Silverlight couldn't provide (such as socket access to a wide range of ports, accelerated 3D graphics, that sort of thing). Moving to Silverlight for an LOB application would generally require you to run all your data access through a web service layer, but that's probably a better way to do it anyway.