To get 7.1 apply for the T-Mobile Beta program the Blackberry Betazone or download one of the leaked OS 7.1 versions from Crackberry.com.
No you do not have to have tethering to get internet access from you blackberry phone to your blackberry playbook. You just need the supplied Bridge App, then the playbook just uses your phones internet access without paying for the hotspot service on T-mobile.
That the beauty of the Playbook and a blackberry phone combo!