Working on a large portal project me and my team faced a lot of problems to get the communication between webparts and binding grids right. To make an end to this, I wrote a universal to use "pattern" to avoid unexpected mess.

If microsoft could only make the connectors available (= onprerender#@$) before CreateChildControls, live would be much easier!

In our scenario, we use one 'Action provider' (save/delete and stuff) and one 'ClassID provider' (to communicate ID's between parts)

http://www.overnight.nl/download/WebPartStages.doc

hope this helps, if not contact me, it will drive u crazy!

Another thing to be aware of, you can only set a validationgroup on CreateChildControls, afterwards, again no error, but it wont render#@ How cute when this depends on the provider#$@ thanks microsoft, not!