It sounds like you are moving from a gamma of 1.8 to 2.2. This will generally make photos look darker and may have a minor affect on your perception of saturation.
How is your monitor calibrated? Mac standard 1.8 gamma? Also, given that the colours are changing it sounds like the photos don't have any profile already embedded. Apple treats pictures without a profile as if they were in Generic RGB which has a gamma of 1.8. When you import to iPhoto it adds the profile Camera RGB - which is a 2.2 gamma version of Generic RGB. Thus, it seems likely your photos are having their gamma affected.
To double check, open a photo that you haven't imported into Preview and 'Get Info' to see what profile it currently has. Then choose tools and 'Assign profile...' and pick Camera RGB. (Note Camera RGB probably won't be there by default. You can find it at /System/Library/Frameworks/ICADevices.framework/Versions/A/Resources and should then copy it to ~/Library/ColorSync/Profiles so that you can use it normally.) Compare the results you get with what happens when you import to iPhoto. If they are the same - now you know what is going on.
There is then a discussion to be had about whether you should calibrate your monitor to a gamma of 1.8 (the Mac standard) or 2.2 (the PC standard). The answer will vary according to your objective.