When you start looking at this sort of thing, you can't really think about it
in terms of controls (.NET specific implementation) or servlets
(java-spesific implementation);

Try looking at it as a web service that takes some number of arguments as
inputs and returns a string for its output.

You're not going to have the luvury of working with postbacks, therefore,
you won't be able to use the even model exposed by the .NET framework (e.g.
you render a button and something happens as a result of that button) since
whatever xHTML you're emitting will not be able to contact the original web
service.

If you do need someething more functional, you could probably try leveraging
AJAX.  no reason you can have a client script calling your web service.