That's only an option if you're shuttering the product of your own will. If you're part of a talent acquisition (which is the case with Posterous and Milk), the purchasing company would have to make that call, so you should really address the blog post to them.

It's unlikely any of them would: they just paid a bunch of money and acquired the team plus source, and even if they don't use the acquired source, open-sourcing it is extra work and/or a liability. The team, on the other hand, may still have faint hopes that the company will do something with their product.

In the cases where it's clear that the team's energies will be focused elsewhere the team could try to include it in the terms of the acquisition, but were I in those shoes I'd probably not want to risk souring the upside or acquisition as a whole.