Whoa, sorry for my lack of response, I hadn't realised that this thread had gone onto a second page :P
Thank you all for your very helpful comments and suggestions.
I'm veering away from Sony & Panasonic more and more, mostly due to the fact that I'm not convinced their four-thirds format will deliver the same longer term satisfaction as a DSLR - they remind me too much of the 'superzoom' compact (Fuji S1800) that I have currently (i could of course be completely wrong and mis-informed). I think Olympus uses the micro four thirds system as well?
I like the Nikons (mainly because one of my colleagues has a D80 and is very happy with it), but am a little worried by the lack of in body AF on the lower end models (D3100 & D5000) and I'm unlikely to find a D90 or D300 within my budget. I know some of you have said that there are plenty of AF-S lenses available, but as for the forceable future most of my kit is going to be bought second hand, I'm a little cautious.
I very nearly bought a Canon 450D last week for a very nice price, but unfortunately timing was bad and the funds had to be spent on other commitments. However, as time has gone by I'm less convinced this would have been a good buy over a 500D or even a 550D? Some of my friends own 500Ds and, having played with them briefly, I have no negative impressions. How much would I be missing out on (besides the video, I'm aware of that) by choosing a 450 over a 500 (or even a 550D if I get lucky price wise)?
In the end money is going to decide what model I get, but I'm getting a good idea of what to avoid and what models to keep my eye out for - and some that I can only aspire to