Desk Top Event
Lately Apple has been updating the laptops all at the same time, both this past February '08 (on the heels of the MBA launch), and again last October. I suspect this is the the start of a trend. Phil S, indicated that the company has several natural cycles now, iPods in September, iPhone in June/July, and as Apple becomes less and less dependent on the Computer part of the company, the computers will get lumped together.
What does this mean for a mini update? Simple, it will get updated at the same time as the MacPro, and the iMac in a desk top event if there's something significant, or all at once as a silent update, like the February '08 update to the MacBook and MacBook Pro if it's just a speed bump.
Just my two cents. I'm voting for mid to late March.