If that Mac Mini goes Core 2 before Santa Rosa and Leopard in April, it won't be until MacWorld San Francisco in January. There will not be a Core 2 Mac Mini before the holiday season.
And really there is no need for it. Sure the Mini is slightly over-priced for what you get for it now as compared to an iMac. However, for whatever it may be used for, and considering it's integrated graphics and slower hard drive, the target market Apple is aiming at with the Mini has no need for a Core Duo to Core 2 Duo bump on the Mac Mini, and so Apple won't do it. Ask yourself why you want Core 2 in a Mini and if it is really a Mini that you want. If you can really convince yourself that a Mini is your best solution but you need the extra power or Core 2, get a used 1.5 solo and mod it to a 2.33 yourself.