what actually is it? and can you get the same effect through photoshop?

I have this facility on my D300 and find it a very useful tool am I right in presuming it just lightens the darl bits and leaves the light bits alone, if so how does it do that and do other computer programmes allow you to do this off camera?

D-lighting works basically the same as the shadows/highlight function in photoshop. Although the camera's function mainly adjust the darker (shadow) areas. The highlights are not really affected.

Also be aware that D-lighting and Active D-lighting are different. D-lighting performs changes on an image after it has been shot. Active D-lighting changes exposure in anticipation of making tonality changes and does it work during the shot.

D-lighting is also a function in Capture NX  with additional options and added flexibility, it is therefore recommended that any D-lighting adjustments is done in NX rather than the camera.

All this wisdom from Thom Hogan's D300 book. Well recommended purchase for D300 owners.

.... and it works a treat!

 in-camera processing only takes a couple of seconds so I doubt that it is as good as the Photoshop method, but it does allow  nice correction quickly with no skills or training.

can I have my camera please...................

congrats simon on winning the first camera

good luck to all for the next 4!

Congratulations, Simon.  Enjoy it.