I always use GPO to deploy printers rather than GPP. As above with Win 7 you don't need pushprinterconnections.exe anymore, but you do if you use XP clients as part of the same policy.
GPP is OK, but I've found GPO is cleaner when it comes to deleting printers or upgrading drivers.
As for setting the default printer using GPO, I do use a GPP regedit for this:
Firstly set the printer as default you require in Devices and Printers. Then open up regedit and navigate to:
Double click 'Device' and copy/paste this into your GPP. For example: