This is a great idea. I hope you are able to reuse some of the work Solution Grove did on the messaging package. You are working in oracle correct? Our package worked in postgres and was delivered via the dotFolio framework, which basically gives you one subsite per user.
My request is to please don't call the package "post-it". I think "post-it" refers more to little yellow "stickies". There are a number of web services that let you post stickies on pages (google "Stickies"). This would also be an interesting feature to add to OpenACS but its a completely different one.