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The consideration isn't really a minor refresh but Ivy Bridge, the next Intel chip which is imminent. This iteration brings an improved fab process (22nm down from 32nm) which means faster/cooler/lower powered chips (take your pick) which will have more impact on notebooks than desktops.
More importantly a much improved HD4000 GPU is due (probably 50% faster than the HD3000), which might be better for Apple than the end user. The reason is that Apple might be tempted to drop the dedicated GPU from more models, and even a 50% faster HD3000 won't come anywhere close to the performance of a Radeon 6750, especially on a high resolution screen. It'll be interesting to see what Apple does, more so on the notebook side.
Anyway, who knows when the new machines will come out. I'm guessing new MacBook Airs in the next 8 - 10 weeks, followed by new iMacs before the end of June. But I'm guessing, and my guess is no better than anyone elses.