If you care about designing web services and believe in the 'share schema and contracts, not class' mantra then you'll be interested in XML versioning.  I mentioned pre-PDC that this was a big issue.  At the PDC Doug Purdy presented one approach that Whidbey Indigo will use to enable versioning with web services.

Just this week Dare Obasanjo has published 'On Versioning XML Vocabularies' and tonight I noticed that Dave Orchid from BEA also attacks the topic with his post 'Providing Compatible Schema Evolution'. 

In fact Dave Orchid's whole site seems excellent (it's my weekend homework since I've only recently subscribed and his feed doesn't contain the full articles so I can't read him on the train).  One good point I noticed was in this post:

'Schema authors tend to make tightly coupled Web services because they can't fully version or evolve them'. 

Avoiding this problem will be key if web services are to realize the promises they are offering today.