We gave Seamless a go, and for the most part it works as advertised. Once both devices are on the same wireless network, you add your Mac from the Seamless settings menu, and that's really all there is to it. You can start a song on either end and have it continue on the other -- the transition works perfectly both ways. All it takes to make the magic happen is a tap of the "Transition Music from / to Mac" button. One major limitation, however, is that the same track must be present both on the iPhone (duh) and on the corresponding Mac. We'd love some sort of reverse AirPlay solution that would let us play directly from iPhone to a Mac as our output, but for now this is the closest we'll get. Oh, and we should mention it's only available for Mac -- sorry PC users.
There's a definite cool factor to hearing a track fade out on one source while fading in on another, and since they're synced it doesn't sound janky or cheap. We still don't have that Continuous Client for messaging and networking we've been waiting for, but this could definitely be described as something similar for music. Seamless is certainly a cool concept and it behaves as described, but we just wish it had a more native implementation. Being able to hit a "transition" button from within the iPod app -- or better yet, in the iOS task switcher -- isn't that much of a stretch, and maybe Apple has something in the cards for iOS 5. Still, if you're a major music lover who just can't stand cutting songs short, you may want to head on over to the iTunes Store link below and experience it for yourself.