The cable adapter converts the IR from the Tivo wands to an IRDA signal that your cable STB will understand. It connects to a socket on the back of the Tivo. It isn't a general purpose IR>IRDA convertor like the RedEye you mentioned in your thread here
http://www.tivocommunity.com/tivo-vb/showthread.php?t=278494 and
http://www.tivocommunity.com/tivo-vb/showthread.php?t=278506
The RedEye will convert IR from your pronto or Tivo, the Tivo adapter works for Tivo alone. If you want to control your cable STB with both then the redeye would be better. There are two buts...
Using the cable box while Tivo is recording will mean that Tivo records what you were doing not the programme you want. Once you have Tivo you will have less reason to use the cable box directly as the Tivo guide is much better and you will find you rarely watch live TV.
Using the original cable remote and the Tivo (via a Tivo cable adapter) can make Tivo channel changing less reliable and it is best avoided. Depending on how the Pronto emulates that remote it may have the same effect.
I use the Tivo adapter alone and have the cable remote out of the way - I only ever use it if the cable box has put up a warning message mid recording.
I also restart my cable box once a day to improve channel changing reliability and clear STB messages when I'm not around. See my signature for details.