So you are saying ANY SLR user should immediately go and buy at least a 50D or D90 or they are wasting their time ? A 50D is nearly three times the price of a 450D, the D90 is three times the cost of a D40.
Any canon can use any lens, the Nikons almost any lens, I don't see your logic Im afraid.
What exactly is (say) a 50D going to give someone who is 6 months in that the 450D wont give (same can be asked of D90 / D40). From reading alot of stuff the main reason for upgrading the body is 1) gone pro 2) new toy syndrome.
As I said it depends on your aspirations. I can only asume the majority of people move into the relm of SLRs because they want to improve the image quality they're getting, or to take advantage of a wide selection of lenses. If it's the new toy syndrome, I don't know, sorry.
I've no idea where you get your info about the main reason for upgrading the body is because of going proffesional, but it's definitly not the biggest reason by far.
The people that post on here are continually striving to improve their photographic skills, it's the nature of the game. As any musican or painter worth their salt will continually seek higher things. There comes a point when the only way to do that is to upgrade their camera. That's not to deny the fact that the really skillfull out there could turn out sheer beauty on any point and shoot. I'm a mere mortal though and need help.
To be more practical and to address the point about the 450D, it is very noisy at 400 ISO, perhaps there are other cheap entry level cameras that are'nt.
As I have found that there are more occasions when I'm shooting in poor light conditions than in good, it's important to me to have a camera that at least works well at 400 ISO. If I use 800 ISO the photos are too noisy for any real use. As it only goes up to 1600 in jumps of 100, 200, 400, 800, 1600 it hasn't even got the scope I need let alone the quality.
Using, for instance the 100-400 or the siggy 50-500, my shutter speed is reduced to useless, unless I boost the ISO and therefore the noise. Another small point is the size of the thing, on these lenses it virtully disappears. I'd like something that feels a tad more substantial.
This is my personal experience, and my particular needs from this camera. Yours may well be different.
As you are likely to spend twice or three times as much eventually on lenses as you do for the body, why buy a camera because of price as the main consideration.
As you say any Canon can use most lenses, some are designed to use them better than others is the logic I'm using.