Lawmakers warned this week that escalating tensions with Russia may leave the U.S. without ready transport to the ISS after NASA retires the space shuttle fleet in 2010.
The space agency does not expect the shuttle's replacement, the Orionan Apollo-like craft being developed as part of the Constellation programto be ready to fly until 2015. NASA's plan was for the interim was to use Russian Soyuz craft to send up crew and cargo to the $100 billion station.

It's an interesting scenario to wargame out: If Ivan refuses to send up American astronauts and sticks to a Russia-only crew, does that mean that they'd be guilty of the first documented case of space hijacking?
That said, Russia will probably honor the agreement. They'll want to avoid the natural influx of funding Congress would send to NASA to fast track Orion or keep the shuttles running for 5 more years.
Lucrative business, spacelift.
--John Noonan