If you find a model that you like now, can use now, and can afford now, buy it now. There is no guarantee that the next model will be better. It is not safe to assume that Apple's development efforts are based on the needs of photographers. As we have seen, anti-glare screens come and go (and come again). So do eSATA ports. Additionally, some are predicting that the cost of memory will rise significantly in the next year, so newer models may cost more while becoming less functional for serious photographers.
I'm not predicting that will happen, but who knows? A good rule of thumb is: Those who know don't talk, and those who talk don't know.
Apart from hardware advances, it always seems to take a while for software functionality to catch up with new hardware capabilities. Thus it is likely that some major advances in hardware will have no real world benefit for quite some time. For example, although hardware has had multi-core processing for some time, Photoshop still doesn't utilize it like it could. It's like driving on the Autobahn in a car that is limited to traveling at 60 MPH.
I also agree that diglloyd.com
is one of the best sources for useful information about the technical benefits (and glitches) of Macs for photographers.