Most of those PSP / PS3 / 360 revisions aren't worth mentioning. They're different SKUs, but they are hardly a notable upgrade, not to such an extent that you potentially hold out / sell and re-purchase the hardware. The slim versions of those systems are noteworthy, but not some minor adjustments, like increased harddrive size or button changes. Bigger harddrives become cheaper for Sony and Microsoft to order than smaller as time goes by, which is why you see new PS3 and 360 SKUs, but they've only had one real cosmetic change.
Alternatively, Nintendo will put out a fairly standard model to begin with, improve on dramatically in the first revision, and then continue messing around with it from there. If I'm not mistaken, didn't the DSi completely change the interface and add in an online store? That's pretty significant, and something you can't take part in if you own the DSL, which itself was a big enough upgrade to ditch the original model if you owned it. Nintendo tends to do this with far more frequency than everyone else, too, which is why "will wait for a revision" mentality is so common.
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Simplicity is the ultimate sophistication.