Smoke screen? Distract? Strange concept.
Canon released the 50D 6 months earlier than expected probably because they feels they need to stay competitive. They did release xxD models 12 months once before.
If anything, the 5D2 is going to slow sales of 50D, with people waiting to see how the 5D2 will perform before deciding on the 50D. Strange strategy on their part, I feel. But perhaps they think the two market is distinct enough, and they definitely need something to compete with the other full frames in the market.
Yeah, strange concept. Beats me. But from a marketing point of view, I think it makes some sense.
It's not just about staying competitive per se. Otherwise, we would see Canon releasing every DSLR model every 12 months. *dread* But it's some fear on Canon's part I sense. I may be wrong.
Cos Nikon is coming stonger than b4. Their noise control in the latest mid level to pro models is already on par with Canon. Nikon used to be noticeably behind in this department. Not to mention, Sony, a non-player in DSLR camera not too long time ago, is coming really strong. Even some pros have no doubts to ditch Canon over Sony.
So what should Canon do? Of cos play the megapixel game. Get ahead in the game with 15MP, more than that found in D90, D700, D3. (But serious, is this what we photographers yearn for over other more urgent features desired? Emphatic NO.)
And while Nikon has long got a much better quality LCD screen, Canon had till then given us a much lower res LCD screen on practically all their DSLRs. So how? Then comes the overdue better LCD screen in 50D. And since they've already designed the super high ISO electronics for the 5D II, might as well throw this part in also. So 50D is born. But unfortunately, the super high ISOs certainly needs more fine-tuning for an APS-C sensor. (Better than nothing, I hear some 50D owners retort? Yeah, if you think that way.)
So in that sense, I feel the 50D is really like "smoke screen" to stave off potential shoppers from buying their competitor's DSLRs or switching camps. It's not so much a conscious effort by Canon to really come up with a convincing upgrade from the 40D.
Like I said, 50D is like a curtain raiser to the 5D II, never mind they are of different "genre" in specs. Of cos, all these are my point of view, slanted towards marketing. I admit I've been very pedantic here, analysing their marketing strategy. Should really be going out to Shoot! [Guess I'm bored now

].
On the contrary, you can view 50D as another one of Canon's great DSLR lineup to be introduced yet again. But I don't quite think it is so. To me, 60D is the one to watch out, for a WOW upgrade from 40D, where they'll likely make a more convincing improvement in super high ISO noise control and hopefully, include a video mode too.