Because this was very requested, today I've uploaded a patch Mint Menu in our MintMenu PPA, which includes a link to the Ubuntu Software Center.
In case you don't know, we maintain a PPA for easily installing MintMenu in Ubuntu. However, the original MintMenu does not include a link to the Ubuntu Software Center (I've named it "Software Center" so the name doesn't get truncated) so the only way to launch USC when using MintMenu was from the command line. But this changes with the patch today (nothing major, just changed 2 lines of code) - as you can see in the screenshot, you can launch Ubuntu Software Center directly from the MintMenu.
I also took this opportunity to upload MintMenu packages for Ubuntu 11.04 Natty Narwhal so if you're using Natty already, you can already use MintMenu (but I haven't tested it on Natty yet).
To add the WebUpd8 MintMenu PPA and install MintMenu, use the following commands (remember, this PPA is for Ubuntu only, and not for Linux Mint users!):
sudo add-apt-repository ppa:webupd8team/mintmenu sudo apt-get update sudo apt-get install mintmenu
Then right click your Gnome Panel, select "Add to panel" and add MintMenu. If MintMenu doesn't show up on the applets list, try restarting the Gnome Panel(s):
MintMenu is also available in the main WebUpd8 PPA!
Thanks to WebUpd8 reader Joe for the MintMenu patch!