Pentax K7 is a advanced amateur level camera. Strengths are its build quality (mag alloy body; weather sealing and ambient operating range). Buttons for fast access are avaliable on the body w/ little need for menu diving. 100% viewfinder. In camera HDR. All K mount lenses (read cheap and good but mainly manual focus) are compatible and usage of these have been considered in the user interface. In camera shake reduction does mean that any lens mounted can benefit from it. High ISO JPEG performance is a mixed bag. Detail retention is very good at the cost of noise. NR can be done off camera btw, if that is ever a concern. RAW high ISO performance is comparable to most cameras of its generation. It suffers the same problem as the D90 though, as the new K5 looks more attractive with the newer specs. It all depends if you shoot often at high ISO.
Support from the local distributor (EMJAY) has been good so far. No complaints AFAIK. Check out the CS Pentax sub forum for more info if you are interested in this aspect.
D3100 would seem good in terms of performance. Almost same as a 550D in terms of the 'creature comforts' part.
D90 is a bit old at 2yrs. Most of the new cameras have benefited from newer sensor and NR improvements, new features as well (eg. HDR, interface refinements). Not that it is unable to take good pictures, just that imo if you want to spend the money, you can get a newer cam with the new whiz bangs (and maybe be immune to camera purchase poison for the next 2yrs). It does depend if you shoot often at high ISO.
That D7K just looks toooo... good
A55, imo high iso jpeg has too much NR applied. RAW is good and very comparable to the new cameras on market. Nice size. EVIL camera comes with its own pros and cons so make sure you try out and be comfortable with using one (Eg. viewfinder resolution; viewfinder brightness in dim and bright conditions; refresh rate) .