Sorry, my response was typed about the time you were posting your second reply, so I didn't address any of your second question...my own input here would be to consider them all. Honestly, that's not a blow-off or standardized response - it's true - it doesn't matter whether you go with Canon, Nikon, Sony, Pentax, Olympus - all make good cameras and better cameras, all will easily meet your needs, and all will give you room to grow and learn.
Depending on if you have a 'hard' budget number, I'd say look at: the two Nikons you mentioned, or the D5100 if it fits within your budget as it has some nice improvements over the other two; the Canon T2 or T3 - the T3 is a nice improvement, but the T2 is still plenty capable; the Sony A500/550 if you can find them on closeout - like the D5000 and D3100 these are last-years' models and share the same sensor, otherwise look at the A33 or A560/580 if those two fit the budget; Pentax KX (along with the Sony A500 & Nikon D5000, this too shares last year's Sony sensor and would meet your needs well) or the newer KR. Possible other considerations might be the mirrorless cameras, if you want to go smaller or lighter but still have interchangeable lenses - look at Panasonic GH or GF1, Olympus Pen EP-L1, or Sony NEX3 or NEX5.
Don't worry about megapixels or brand name - both are the least important factors for entry-level DSLR needs. Worry about ergonomics - how they feel in your hand - control layout. Sensor-wise, any of the above will be as close as not to equal - all good in low light and with decent dynamic range - maybe some other features might be worth considering if they are important to you, such as video specifications or abilities, live view systems and abilities, stabilization types or in-body systems, or special features like built-in HDR, panorama, stacking, bracketing, etc.