Reality

Many different choices Web Services come in a lot of flavors, not all of them interoperable. Simply stating that a system will be built using SOAP or BEEP isn't enough information to define what the project is really about.
Different architectures XML-RPC is very simple remote procedure calls, while SOAP can do RPC or document exchange. BEEP is really a protocol for protocols, and other communications approaches like Jabber offer even more options. There are lots of choices and styles, not a single simple approach.
Network effects The effectiveness of Web Services depends largely on how many people use them, much like XML. Anything Microsoft or IBM does, for instance, will likely develop large backing. That can be good or it can be bad.
Untested It's early yet. While the pieces appear to mostly work, their interactions and evolution are not yet explored in depth.