Preview at WWDC likely in June, followed by September launch.
Intel Mac mini Upgraded to 2.16GHz Core Duo
This appears to confirm that the Mac mini's Intel processor is upgradable. Disassembly pictures posted last week revealed that the Mac mini Intel processor is socketed like the Intel iMac, suggesting that a simple processor replacement could be performed.
I picked up an Apple Mini Mac for $599 and Fry's Electronics today. It was the low end unit with plans to swap out some of the guts for a mega upgrade that would be an unavailable configuration from Apple.
For those with access to Yonah chips or prefer to purchase from an online retailer but did not have a platform to run it on, this is very cool interim machine to check out.
The author posts screenshots showing Before (1.5GHz) and After (2.16GHz) system information screens documenting the upgrade.