This is updated often so check out the price of what you want before you buy
http://www.camerapricebuster.co.uk/cat4.html
Just wanted to post this extract from a poster on DPReview. He tallks about the Nikon slr's but it is true for other makes with entry level and prosumer slr cameras i would imagine.
The D3100 is aimed mainly at people who are buying their first DSLR and want a bit of hand-holding and not too many options to deal with. It's missing quite a few features and options that a more experienced user might want that the D90 has.
Here are a few things based on what I'm seeing in the preview here that I can come up with quickly that the D90 has that the D3100 doesn't:
In-body focus motor, so significantly more auto-focus lens options
Dedicated wheel for adjusting F/stop
Assignable function button on front
Depth of Field preview button
Exposure Delay mode, mirror lock-up of sorts
More dedicated buttons in general
Top LCD
Faster continuous shooting, 4.5 fps vs. 3 fps
Pentaprism viewfinder, larger and brighter
Hi-res monitor, 920,000 pixels vs. 230,000
White Balance options include Kelvin temperature, 3 White Balance pre-set memory settings
Center-weighted metering adjustable, more options
More image quality options, can shoot NEF plus JPG Normal or Basic
Battery grip available
You can set up custom "My Menu" for often-used menu items
There are probably a few more too. I would miss most of these if I was shooting with a D3100. The D3100's higher resolution will make diffraction softening a bit worse when you use smaller F/stops, which is a big negative to me. NEF's will require more memory storage than the current 12MP bodies.
As far as I'm concerned, 12MP is about the sensible limit for an APS-C sensor. Beyond that, you're seeing diminishing returns.
John