I've seen some reasonably plausible theories that ice ages can in fact be caused by sudden spikes in planet-wide temperature. Supposedly, data from the Greenland Ice Sheet cores supports this theory, showing wild temperature swings immediately preceding the end of an interglacial period. (Wish I could find the link to that article). Perhaps a combination of increased cloud-cover through increased transpiration, and decreased salinity of the oceans through ice-melt, is enough to shut down major ocean currents and dramatically flip things the other way.

I know that it would actually be far more catastrophic than a greenhouse Earth -- and perhaps it's because I've already spent too much of my life sweltering in deserts -- but I've always kind of relished the prospect of an ice age. There's just something deliciously ironic about the image of once-poor low-laying island nations enjoying their newfound beachfront property, while Detroit sits under an ice sheet half a mile thick. :-)

Cheers,

Nathan.