Not that long ago, FUGGER, owner of the XtremeSystems.org site, reported that the processor inside the Mac mini could be upgraded by simply swapping it out and replacing it with a better one. This lead to a great deal of enthusiasm and tinkering with the minis. However, the implication of the easy upgrade was not anticipated by anyone.

It seems that the upgrade ability goes beyond the Yonah processor, and even the soon to be available Merom will work.
"It is possible to drop in Merom into Mini, one hell of an upgrade path. Confirming this is a working combo, just drop the chip in.", FUGGER writes.

While further details are unavailable, probably due to non-disclosure agreements, the Merom is one big step for a Mac mini, being a mobile 64-bit processor due in late 2006.
This new processor is said to offer 20% more performance than the current Core Duo, without "eating" any more of the battery life. Reportedly, when it comes out, it will have 4MB L2 Cache (up from 2MB on Yonah).

The backwards compatibility of the processor has also been advertised by Intel, when Mooly Eden showed a benchmark pitting a Dell Core Duo system against the same system with a Merom processor. Eden literally swapped out the Core Duo CPU and stuck in a Merom processor.

This is good news for anyone who was contemplating the upgrade of the new iMac and mini. However, portable users will likely feel left out, as the processor in the MacBook Pro, and very likely the one in the upcoming MacBooks, is soldered to the motherboard.