Is the Lego 8884 IR Receiver compatible with earlier IR Speed Remotes e.g. from set 7897?
I think RC devices are not compatible with PF devices due using different IR protocols.

For instance, see this forum thread for more explanation. Also this cool site.

Summarizing,


RC and PF can use the same rails
RC and PF cannot use the same remote controllers
RC and PF uses different engines:


RC places it in bogie (as 9v do)
PF places it in body