Just hands-on the 1kD (1000D) and 450D side by side at the Canon showroom, here are some of my brief thots (note: this ain't a full in-depth review)
Side by side comparison: Both cameras remarkably similar in size, form and function. No real difference in weight...the 450D is perhaps just slightly heavier but not really noticeable in use.
Handling/build: 450D is slightly better...the 450D rubberized grip is easier to hold on too compared to 1kD's grip. The 1kD's grip feels too plasticky and smooth and it is really uncomfortable to hold for long periods of time if you have large hands (like me). I can't imagine holding it with a large L lens attached. One irritating aspect about the 1kD is that the LCD can't seem to be turned off manually, and there is no sensor that autodetects ur face and switches it off automatically unlike the 450D. Build wise I feel the 450D is better off, if only marginally.
Viewfinder: This is where the 450D trumps the 1kD hands down. This alone I feel is worth the price difference between the two cameras. The difference is quite a lot! The 450D's vf is clearly much brighter, bigger and clearer than the 1kD's. With the 1kD I felt myself squinting thru the vf...it was still as dim and tiny as in the 400D. If you do more than 80% of ur shooting thru vf and not Liveview, this is a very important factor and I would recommend 450D over 1kD, especially also if you wear glasses.
Shooting/buffer: For JPG Large Fine mode, the 1kD managed to do about five to six frames in continous shooting mode before the buffer jammed up. The 450D is better in this aspect...about 9 frames continous before buffer full. In RAW mode the 1kD fared very poorly...only about 3 frames can taken before the buffer full and the writing took forever (maybe coz of the card used also lar...not a hi-speed one)
LCD: again like the viewfinder, the 450D's is bigger, clearer and better all round.
Conclusion: Unless you're really really desperately cash strapped or refuse to shell out the ~$200-300 difference between the bodies, I would feel that the 450D is overall a much better package than the 1kD and thus it gets my vote. The swaying factor for me was the viewfinder issue - I simply hate small dim viewfinders...but again YMMV...whether u feel the ~$200-$300 in savings is worth for you, the 1kD is an decent body...just not fantastic. To be honest, it feels more like a 'rehashed 400D' than a new body.
), and from the specifications of both cameras, the 450D is a better camera. So if you can get it at a good price, your choice is clear.