I've upgraded several original CoreSolo and CoreDuo Mac minis by adding a Core2Duo, upgrading the firmware (so they'll support 3 gigs of memory) and installing Lion.

It is so incredibly easy: put the mini in FireWire target mode, connect to a Mac that can install / run Lion, install and update Lion using the other Mac, then just sudo rm /System/Library/CoreServices/PlatformSupport.plist. It'll boot and run just fine on the mini.