I would imagine they are trying to get people back into believing in the device after the whole reboot debacle that ensued for so long. Now that Gingerbread is rolling out for it and those issues should be resolved, everyone should be happy campers. It is unclear to us if Gingerbread is pre-loaded on the devices that are shipping or not, but often times a newly activated device will get the update nearly immediately.
If your sights are aimed at picking this device up, head on over T-Mobile.com and get your order placed before the offer is removed.