Last week on April 8 Apple released Xcode 6.3 including the SDK for iOS 8.3. We have tested the current versions of Titanium Studio and Appcelerator Studio as well as our CLIs and current SDK for these new versions and found no problems. The same is true for the beta versions of the upcoming 4.0 GA.
If you do run into a problem you believe is caused by an incompatibility issue, please create a ticket on JIRA including as much specifics about the error as you can.
The previous release of Xcode and iOS included WatchKit, which we have phase 1 support for in beta. We will have a detailed guide ready at GA.