Regarding Mindstorms NXT to Mindstorms RCX communications, HiTechnic.com is preparing to announce something new, but you're getting a sneak peek here first, thanks to Steve at HiTechnic!
The HiTechnic Bridge will allow the NXT Intelligent Brick to communicate with the Mindstorms RCX Brick.
The HiTechnic Bridge will use an Infrared (IR) Transmitter and Receiver (connected to an NXT sensor port) and that's how it will 'talk' to the RCX through its IR port. Full bidirectional data transfer will be supported and can be initiated by either Brick.
Look for it in August, to coincide with the NXT system retail release. (And check the HiTechnic website for more info very soon, including pictures.)
Thanks again, Steve, for the preview.