The upcoming Ubuntu 11.10 release (Oneiric Ocelot) will drop Synaptic Package Manager from default installation in favor of Ubuntu Software Center.
This has been foreseen since Canonical has spend their effort in developing and improving Ubuntu Software Center as a convenience one-stop center to install free and paid software alike.
Personal Thoughts
Personally, I still prefer to use Synaptic Package Manager over Ubuntu Software Center as it is still doesn’t offer the same number of options that I used to get from Synaptic (ability to search by name, description, ability to test the fastest mirror).
But I believe the changes that Canonical and Ubuntu community made might make it easier to streamline the looks and functionality of package manager into desktop environment (Unity) regardless of the computing platform.
