I use LR and I also use CS. I have no problems with round tripping from within LR to CS and back again.
I am asking myself the same question “Should I upgrade to CS3 or not”. It is true to say that, from what I have read, the integration between the CS3 and LR is tighter than with CS. CS3 knows about LR and will pass certain information to LR. I do not know what you can do with this integration though.
The other reason to consider CS3 is that it will work with Windows Vista. As I understand it, neither CS or CS2 will not be patched or certified to work with Vista. Having said this, I am running CS on Vista and have experienced no problems (although I know other people have).