When two proteins are found to co-immunoprecipitate does this indicate a distance between the two interacting proteins? In broader terms, are their limits to the detection of interacting proteins through co-immunoprecipitation? Also, if proteins interact in a pathway (anywhere in the pathway--say protein A does something to affect protein B), do these co-immunoprecipitate? What methods-besides FRET- are useful to prove that protein a and b interact?

Hi Cynthia,
If you perform a Co-IP study and manage to IP several protein together it meens that there is an interaction between the proteins that you CO-IP'ed. It could be in any part of a pathway. If they are interacting you would see them in your western blot.
You must bare in mind that depending upon your lysis buffer you could disrupt the link between the proteins. So even if they are in a complex and you are using a harsh lysis bufer you could disrupt the bond between the proteins. For CO-IP it is usualy beneficial to use mild lysis buffer. I.e. 20mM Tris, 150mM NaCl and 0.1% NP-40 (pH7.4)
Some times people are using cross linkers I.e. DSP to prove protei-protein interaction.