Hi Russel, great question!
The problem with in-camera stabilisation is that the sensor can only be shifted by so-much, regardless of the lens you attach. Since the compensation required to counteract camera-shake increases with focal length, there comes a point where the in-camera systems just can't keep-up. In contrast, if it's lens-based, the manufacturer can equip it with the tools to do the job.
This only really comes into play at much longer focal lengths though, but the great advantage lens-based systems have at any focal length is the ability to show you the effect of the stabilisation as you look through the viewfinder.
If you're framing at, say, 200mm, any wobbling can make composition quite tricky. It's wonderful to see the image stabilise through the viewfinder the instant you half-press the shutter release. It makes the composition very easy.
I remember testing the Sony 75-300mm lens on the Alpha A100, and when zoomed-into 300mm (working at an effective focal length of 450mm), I found it hard to compose the shot. The anti-shake system may have resulted in a sharp image at the end, but it was tough to line-up the shot as desired.
Ultimately it's a personal preference though, and models like the forthcoming Olympus E-510 will certainly level the playing field, as you'll be able to preview the effect of its in-camera stabilisation thanks to Live View. The in-camera stabilisation system of the SP-550UZ bodes well for the E-510 as seen in our video at:
http://www.cameralabs.com/reviews/Olymp ... age7.shtml
It'd be great to hear from anyone who's using a camera or a lens with stabilisation. Are you pleased with the results? Do you think it works well in practice? Let us know!