I know this blog is dedicated to the Nikon D3000 and it's features, prices, etc. but I found some great information from nikond3000review.net on the NIkon D3100. Enjoy!
Additional Features & Upgrades Now In The Nikon D3100
- 14MP CMOS sensor – A large step up from the D3000′s out of date 10MP CCD sensor
- 1080p24 video – The D3000 doesn’t have 1080p video and the quality is better than the D5000 and D90
- Autofocus while filming -This is a first for any Nikon DSLR; previously you had to lock focus pre-recording and then tweak it manually
- Live view mode – Comes hand in hand with the new video features
- Automatic chromatic aberration correction – Again this does not exist on the D3000. It’s a big improvement that fixes fringing in your pictures before you save them; it was previously only available with the D90 and D5000
- Customizable function button – Makes life a little easier
Bottom-line: While the D3000 is about $100 – $150 cheaper than the D3100, the addition of a better 14MP sensor and 1080p HD video make the Nikon D3100 a clear choice if you can afford the extra bump in price.
Again, the bottom line says it all, if you can afford the extra bump in price it's a good alternative, however it's basically just as good as the Nikon D3000 for the price.