For example, I know that the WCF style web services don’t rely so much on Session/Application/Request/Response as web services did. Are there any other gotchas I should keep in mind when writing web services today with the intention of making them WCF services as soon as my organization lets me use WCF?
I don’t think I’m interested writing 2.0 webservices then using WCF’s compatiblity mode, because that would be all the complexity without so much of the benefits of the new model.
One suggestion that comes to my mind is to build your web services using Contract First Approach due to its conceptual similarities with wcf message/data contacts. Besides that MSDN has the following articles that are good guides to embark on a migration mission-