I noticed this in 2008 just before the new one-piece enclosures first broke cover and then again last year when the line-up was last refreshed. Here we are in 2011 and once again the number of MBP threads tells me that new models are imminent.
A number of factors seem to support my theory:
1. Sandy Bridge processors are out in the wild. Paul Otellini of Intel says he thinks that they are a significant step forward versus other recent processor upgrades, (of course, he would say that, but the data makes a solid case). Other manufacturers are releasing machines that incorporate SB processors. Apple has to follow suit or risk falling behind. This is especially important considering that Apple's machines are more expensive than many competitors, and also because it wants to grab a slice of the corporate market.
2. The current Core 2 Duo architecture is starting to show its age. It wasn't replaced last time around, when Nehalem processors first appeared because the arithmetic of Intel processor plus third-party graphics chip times competitive pricing didn't add up below 15" machines. Arguably, with Sandy Bridge, Apple has several new options to deliver new value machines that provide superior processor and graphics performance at reasonable prices.
3. Since 2008, the advent of movies on iTunes has chewed up a lot of disk space. Suddenly, all our hard drives are full. Our batteries are worn out too. instead of just replacing the HDD and battery with new larger-capacity ones, quite a few people are thinking: why not replace the whole machine? I know I am. As Airplay is used to stream content from one source computer to other devices, large capacity disks will be increasingly essential.
4. Apple's clever incorporation of SSD chips in the latest MacBook Air models shows that solid state memory's time has come. Instant on and greater reliability versus traditional spinning platter HDDs is likely to drive demand. But I guess it is going to be some time before the price of a 1 TB SSD approaches a competitive level! (Understatement of 2011 to date.) Given Apple's monopoly of the SSD market, it could do much to make this technology more mainstream and more accessible.
5. The writing is on the wall for the ODD. Ditching the DVD drive frees-up space for other goodies. The App Store now makes it easy to download software. We're also seeing USB memory sticks being used to store data very effectively. So who still needs an on-board DVD player? I can count the number of times I've used my DVD drive since I bought my MBP on the fingers of one hand.
Perhaps none of these factors by themselves is enough to suggest a change, but taken together, I think it is only a matter of time. I am pretty sure new machines will arrive within 8 weeks.
Now just hang on a second while a put on my flame-retardant suit...