W3C and OASIS have put out some new recommendations on standards for Web Services. I don't know about you but I'm still trying to get my hands around the existing standards. One of the biggest challenges we have seen is interoperability between vendors. We are using a variety of vendors including Microsoft(.Net), webMethods, webSphere, custom C clients, custom Java clients plus others. All of which seem to have varying levels of support for Web Services.
One of the bright spots is the ease of which webMethods and .Net interoperate. Of course this is assuming you get over the hump of how webMethods works with doc/literal. Many thanks to Mark Carlson for that bit of knowledge.
I haven't found the interoperability between webSphere and anything to be as straight forward. I believe that is mainly due to the way the WSAD IDE does things for you behind the scenes. I admit I'm not an expert with the WSAD but I do fine that it generates code behind the scenes that uses IBM specific classes or implementations that does not always play well with others.