Web Services can be easily developed on OSX. WebObjects is a great way to do it, if you're not already there. But programming in Eclipse will work just fine.
Deployment gets easier under panther, with JBoss and all.
But if you want to code the java beans for your web service, OSX is a great way to do it. Even ProjectBuilder will allow you to develop the web services. There should be nothing to stop you.
---gralem