M is short for Modificata and denotes the half life point of the basic chassis of each model so the M point in recent model history equates to the 360 becoming the 430, 550 becoming the 575 etc. The size and scale of the revisions is determined by the sales volume of each model line hence why the 612 had no significant sheet metal changes during it's "full" life yet the 430 received a brand new engine, e-diff, new interior, revised styling etc.
The 599 replacement will be a reworking of the basic architecture of it's predecessor but it will feature the all new V12 from the FF along with the DCT gearbox from that model.
The forthcoming changes to the California appear to fall outside the planned time window for the M model. I suspect the revisions have been pulled forward to prevent sales dipping following the launch of the 458 spider. The changes sound like they are aimed at tempting existing Cali owners to trade up to the new 2012 model and keep the order book evenly spread on the V8 line across the 458, 458 spider and Cali. I'd be surprised to see any significant styling alterations to the car at this point as it has only been on sale for 3 years.
Current: F50, 246 GT (x 2), 246 GTS
Past: 360M, F430 spider, 430 Scuderia, California (x 2), 458 Italia (x 2), 458 spider, 246 GT