I installed IDC 3 today and here are some observations compared to IDC 2:
1. All settings are set to off/0 when an image is loaded
except Noise Reduction, which is
always set to Auto. You can turn it off and then if you change to a different creative style (vivid, etc.) it quietly returns to Auto.

It is very easy to forget to check that. It should either remember your setting, have a way to set the default to off, or just make the default off like every other setting.
2. I'm very glad they fixed the weird behavior in IDC 2 where sending a file directly to PS was implemented in a terrible, brain dead way. I submitted a bug report to Sony many months ago and also posted on the Sony blog about this and I am glad to see they implemented it very well this time.
3. When you first start IDC 2 it uses about 16mb of memory and IDC 3 uses about 8mb. This is on Vista.
4. In the Settings|Development tab there is a new option to select High speed display. If you select this then it seems the image is updated faster
but it has 2 problems: (a) even though you select it and click on the apply button it doesn't actually take effect until you exit IDC and restart it and (b) if you select this you can only zoom up to 50%. That isn't a big deal, you have to expect some compromises with this option,
but it means that everytime you zoom in you first get a warning window telling you that the maximum is 50% and then you must click OK. It slows things down a lot. It should allow me to tell it not to show the warning anymore.
Overall, IDC 3 is a much slicker and nicer program and I like it better than IDC 2. I haven't checked to see if it produces better/different conversions, but I imagine it is at least as good as 2. I think I read that the DRO has been improved, but I prefer the much finer control using the PS shadows/highlight function which has about 10 controls to adjust the way it works.