Todd Biske pointed out this article by Surekha Durvasula on service versioning. It's a good article and something to give some thought to. Believe it or not you will have the opportunity to have more than one slightly different implementation of the same service. It's not something I really bought into a while back but I have seen the value first hand.
There are some other low level reasons for versioning services as well which have to do with your schema design ie namespace management and your xml schemas. I'll return with more detail on that in a later post. XML schema design and Web Services is an area lacking in attention in my opinion. Doing it correctly is really the foundation of a good service.
Do you need a really expensive governance product to do service versioning? No not really. As long as you have a policy that is defined and manageable, you are on the right track.