OK I have done the same thing with a few pictures from each session. I color correct in Lightroom then open in CS3 and do some fun stuff to them there. I have done it this way for a while and all the sudden I am not able to save as a jpeg. The option is not even there. What changed??

check if something has created an extra "channel" in the channels pallette or if your color mode has been switched to grayscale or some other or if you somehow got into 16 bit mode....off the top of my head....

some actions and things like that make these kinds of changes sometimes...

Kate