Don't know about the 450D, but there have been a LOT of discussions about the 400D, and, apparently, it is impossible to know. There are software out there that can "guess" with around 10-15% certainty on shutter actuations on other makes, but not the 400D, they work for higher end cameras and other brands, even though the results, as I said before, vary. Apparently, for the Canon line of cameras, only with the xD you can know the actual shutter actuations.
I do hope someone comes along and proves me wrong, because I've been looking for such a software for some time now and read everything on the matter there was to read.