Preview at WWDC likely in June, followed by September launch.
Apple Pitches Security of iMessage in Response to SMS Spoofing Issue
Engadget reported over the weekend that it had obtained a statement from Apple on the issue, with Apple simply touting its iMessage service as a more secure alternative to SMS.
Apple takes security very seriously. When using iMessage instead of SMS, addresses are verified which protects against these kinds of spoofing attacks. One of the limitations of SMS is that it allows messages to be sent with spoofed addresses to any phone, so we urge customers to be extremely careful if they're directed to an unknown website or address over SMS.
iMessage is of course an Apple-specific messaging service, and is thus only compatible with iOS devices running iOS 5 or later and Macs running OS X Mountain Lion. Consequently, it is generally not possible for users to entirely replace their SMS usage with iMessage. Apple has also not committed to making any changes in how it handles reply-to addresses for SMS, so it is unknown whether they will be directly addressing the issue.