You can upgrade from Photoshop to the entire Creative Suite but not the other way around.
When Adobe introduced the Creative Suite about 2 years ago, the concept was a cheaper way to get all their software ($1,199 instead of $2,695). They even provided an upgrade price for those that just owned Photoshop and wanted the entire suite. What was not explained well and what CS1 owners are now finding out is that you were buying a bundled package and not the individual applications. What does this mean? Those that bought the first Creative Suite can not upgrade just Photoshop, Illustrator etc. If they just want Photoshop CS2 ($150), they are now required to buy the entire CS2 Suite ($550) and upgrade everything.
Surprise.