Hopefully all the heavy lift for ISS will be done first.
Well - the point of retiring STS in 2010 is so that they've already got the large segments up the hill. All the large components are designed specifically with STS in mind and would require not insignificant modification to be lifted in any other way.
Thereafter, COTS is hopefully going to offer the means to throw smaller pressurised and unpressurised stuff up to ISS. Typically the systems appear to be like a short stocky 'command module' with various configs on top of it to carry whatever is requried, they hope all the way up to a crew capsule. The unpressurised capacity will be for things such as replacement CMG's etc.
Doug