When I go into Help Browser and select examples/Gem/4. video/00. simplevideo pd I get no response. Isight is activated and PD crashes i.e. I have to force quit PD.

Gem seems to have loaded correctly in the control window so I'm not really sure what to do.

Does anyone know what might be happening here?

nope, works like a charm here.
macbook 13" alu osx 10.5.8
pd-extended 0.41.4

you don't have another program open which might access the cam?!

pd main window sayst:
[pix_videoDarwin]: height 320 width 240
[pix_videoDarwin]: SG channnel Device List count 3 index 2
[pix_videoDarwin]: SG channnel Device List DV Video
[pix_videoDarwin]: SG channnel Device List IIDC FireWire Video
[pix_videoDarwin]: SG channnel Device List USB Video Class Videoem/Library/QuickTime/QuickTimeIIDCDigitize
error: [pix_videoDarwin]: SGSetChannelDevice returned error -9408
... you might be able to track this down from the Find menu.
[pix_videoDarwin]: vdigName is Built-in iSight
[pix_videoDarwin]: digitizer rect is top 0 bottom 1200 left 0 right 1600
[pix_videoDarwin]: active src rect is top 0 bottom 1200 left 0 right 1600
error: [pix_videoDarwin]: could not set SG Rect
[pix_videoDarwin]: set SG NormalQuality
[pix_videoDarwin]: using YUV
GemwinMac: width - 500 height - 500
GemWindow Activate err = 0
GEM: GLEW version 1.5.0
GEM: Start rendering