Just to jump in here, Whether or not Enterprise Server will work for you depends on whether or not you have control over both ends of the application (both the server side, and the client side.) If that is the case, than you can publish an enterprise server on your own publicly accessible server, then use that enterprise server as a data source for the client. This allows the client to communicate with the remote server just as if it were a local data source. In most cases this accomplishes the same task as a web service, but uses a proprietary communication protocol (similar to serializing the data back and forth, but much more efficient.) In the situations where it fits, Enterprise Server is very effective, simpler to implement than web services, and much faster in operation.
By the sounds of it, however, you are providing a web service which your customers connect to. If that is the case, than they (your customers) would have to purchase strataframe to encorporate enterprise server support within their own applications. If this isn't what you are looking for then yes, standard web services would be the best answer. Strataframe does not require additional functionality in order to work in a web services environment, so implementing that portion itself would be the same as implementing a web service using standard .NET. There are quite a few docs out on the web that do a great job of outlining that process.
Once you have your web service published, and are parsing the necessary information out of or into the required XML files, that is where StrataFrame can come into play to help marshal that data, using BOs and business logic to keep everything organized and clean on the backend. This is what Trent was getting at in his earlier post. You would create a factory class (just standard .NET) to grab and interpret the received XML file, and that factory class would be where the integration with StrataFrame would begin. We'd be happy to give some specific direction here if you can share some more specifics of your application itself (i.e. what data you are expecting, what you want to do with that data, etc.)
If you do have control over both ends of the application (server and client side), then I would definitely recommend using Enterprise Server rather than trying to implement web services. As I mentioned, it is quite a bit simpler to implement, and is much faster (and potentially more secure) in operation.
Hope it helps
Dustin