When Sprint decides to deploy LTE in the PCS A-F blocks, MetroPCS' and some AT&T phones should be able to(hypothetically) use Sprint's LTE network if provided with provisioning and a SIM(in the case of AT&T devices, data-only would most likely be the only choice available unless there VoLTE). If AT&T decides to create LTE phones that are able to access their 850MHz holdings(but using superset Band Class 26 instead of just Class 5), those devices should also be able to be usable on Sprint's 800MHz LTE holdings.
This Pantech device(P8010) just went through the FCC. It's includes Band Classes 2, 4, 5, and 17. It doesn't include superset Band Classes 25(Class 2 + G Block) & 26(Class 5 + ESMR).
http://www.engadget.com/2012/05/15/pantech-p8010-meets-fcc/
https://fjallfoss.fcc.gov/eas/GetApplicationAttachment.html?id=1687578