I could see some real use cases for OAuth. Especially with regards to the cases mentioned above. People could potentially build apps like AWB and Huggle using OAuth. In general I think this would be a "cool thing" to have for all MediaWiki installs.
As for being an OpenID provider... only one major thought: Having this Foundation be a provider would be a lot of additional server load (It is 100% non-cacheable) without any benefit to the main goal of providing free information.