Definitely most of the big states have vast stretches of legal-to-ride freeway. You'd probably have to ride quite a ways from Philly to find some though.
In Oregon and Washington all Interstate miles not in or near a big city are legal to ride with the current exception of Snoqualmie Pass due to construction.
Here's the guide to sections of highways you can't ride in Washington
http://www.wsdot.wa.gov/bike/closed.htm
This is what crazyguyonabike.com came up with in 2007:
"Allowed on all interstates: Idaho, North Dakota, South Dakota, and Wyoming
Allowed on certain sections of interstate system: New Jersey (Permits granted for particular use and location), North Carolina (DOT may approve opening certain section), Pennsylvania (DOT may approve opening certain section)
Allowed on interstates where no alternative route exists (usually means access is prohibited in urban areas): Alaska, Arizona, California, Colorado, Montana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, and Washington
Access not expressly prohibited: District of Columbia and Missouri
In all other states, bicyclists are not allowed to ride on interstates. However, even in these states, there are exceptions to this rule where bicyclists are permitted to use a particular bridge that is part of the interstate system (e.g. I-66 in Virginia, I-70 in Kansas)."
Considering the square mileage of the states in the "Allowed on interstates where no alternative route exists" and "Allowed on all Interstates" lists then I guess you could safely say it's allowed in most of the US