I occasionally use Picasa on Windows (but I always end up removing it, since I don't really use Windows for photography that much) and I must say that, regardless of whether or not its use of WINE on Linux is a letdown for purists, the overall user experience on Linux is just as good - it's fast (very fast considering the hopeless hardware I'm running it on), pleasing to the eye and every bit as functional as the Windows version (as far as I can tell by throwing in two month's worth of photos and playing around a bit).
Sure, the Windows bits of the UI are a bit jarring (and Picasa itself isn't an ideal photo management tool, given its lack such of features as EXIF editing, etc., etc.), but for the casual user, this is a pretty good first step - using Fedora, I just clicked on the RPM link, got prompted to install it, did so, and it worked without any fiddling under the hood, which is as it should be.
Still, despite the visually impressive slideshows and timeline features, for real photo management it's nothing more than a toy - F-Spot already wipes the floor with it on EXIF handling, and those bits of F-Spot aren't even final.
On Courting Other Platforms With WINE
Actually, WINE strikes me as a pretty good compromise for some things. For instance, there are already rumors of a Crossover Office alpha for Mac OS X, which would conceivably make it possible for me to run Visio natively whenever I get around to buying a MacBook. And yes, like some Linux users, I'm somewhat worried that things like Darwine will give rise to half-baked Mac OS X applications on Intel, but I'm confident that both users and developers will - if given the choice - choose going fully native over using shims.
There is a third way, which is using things like Mono or Qt to ensure your applications are portable - at different levels - from the outset, but there's nothing like going native (Google Earth is a good example of using Qt instead of native Cocoa widgets that sort of works, but would do well to go fully native).
Obviously, there is a big difference between things like WINE, Mono and Qt, but I don't think there is any need to panic when a popular application gets ported to another platform by using any of them - native applications will always have an edge, in Gnome or anything else.