TN Panel = Laptop screen
The difference between the new and old 20'' iMacs is different LCD panel types.
The white 20'' iMac uses an S-IPS panel. This panel type is usually more expensive to produce, and is the type of panel graphic artists seek because of the extended viewing angles, among other features.
The ALU 20'' iMac uses a TN panel, which is also found in almost all laptop screens. TN panels are literally cheaper than S-IPS, but it doesn't necessarily make them "worse" (as millions of laptop users, some even graphic designers, can attest to)--it depends what you're using you're computer to do. For instance, while TN panels have limited viewing angles in comparison to an S-IPS panel, the TN panels generally have better response times than S-IPS panels.
In short, if you're not going lots of graphics work, you'll barely notice the difference, unless you shift your viewing angle frequenlty/it's pointed out to you in a forum like this. If you sit centered to your monitor, the TN panel will look excellent (been to an Apple store?)
No LCD panel technology is perfect, they all have their pluses and minuses. The old 20'' had a more expensive LCD panel, but the new 20'' iMacs are cheaper. So, there's the trade-off--it's not like Apple is charging the same but using cheaper parts.
Rest assured though, Apple is not foisting inferior panel technology just DIFFERENT panel technology, which helped them reduce the cost of the machines. The white 17'' iMac also used TN panels. So, that about sums it up
