I have been wrangling with color profiles in Adobe Lightroom and Google Picasa for some time, and I hope that what I have discovered will help some people.
First off, if you are using Lightroom, or any major RAW editor if my information is correct, you will be working in the ProPhoto RGB colorspace. This is a very broad color space that is ideal for tweaking the colors of rich photos.
Before this became common, the most common was AdobeRGB, a color space that was created as a professional alternative to the sRGB color space that is the default of Windows and most other programs.
Unfortunately, not all programs play nicely with AdobeRGB, much less ProPhoto RGB. Picasa is one of them. I have found that a picture may look perfect in Lightroom, but like total crap in Picasa. It's because Picasa does not, apparently regardless of the color profile in which Windows is working, display ProPhoto correctly.
When displaying the images online, Picasa seems to be able to figure it out and provide correct photos, but not in the desktop software. This is doubly odd because Windows Photo Viewer seems perfectly capable of correctly displaying images that are in both the AdobeRGB and the ProPhoto RGB color spaces.
This is beyond annoying. I hate having to jump around in color spaces as opposed to using a single space from process to print. It also means that, when posting online, you have to specifically restrict your images to make sure that they display correctly in all of the most popular viewers, of which Picasa is certainly one.
I suppose that it's not the worst thing in the world, just make sure that whenever you are displaying images directly from Lightroom or ACR, encode them in the sRGB color space to ensure complete compatibility. But c'mon, Google! Even Microsoft managed to get it right, and they're idiots.
UPDATE: The color rendering problem also exists within Chrome. The following screen shot was taken of the same photo, on the same page of this blog. On the left, Firefox, on the right, Chrome. Seriously, Google, what the hell is going on?