I haven't heard anything, but IMO, Cary is trying to be like the Roman Empire and slowly take over the entire region. Maybe they want to put up more of those hideous, useless, electronic signs (yet still have the most ridiculous sign ordnances ever). I've also heard rumors that they want to take over parts of Holly Springs. But, as far as I know, they're just rumors.
I really don't know where you'd start. Maybe attend a town meeting and raise the question to the board?
If they do take over, you could always move to Fuquay. We still have Holly Springs between us and Cary and as a bonus, you get to live in a town that no non-NC resident can pronounce.