If I am a telecommuter, say I live in Florida but work for a Company who's offices are in Pennsylvania, am I subject to PA state and local taxes?
Depends on the state, but in the case of Pennsylvania, it looks like you may be subject to PA state taxes (at least, local may depend on locality) depending on the circumstances. Several states - almost entirely states near NYC - tax income derived from employers located in their state regardless of where work is performed. Deleware, Pennsylvania, New Jersey, New York, Connecticut, and Nebraska all tax this way (as of current time).
See for example this rather technical document discussing the laws behind the issue.
Also see this less technical document which gives an overview of some of the problems you may face.