Being a big boy I unabashedly disclose that every time I hear SaaS I go "why all this noise, what's the big issue?"
It is after all only a "distribution and maintenance method" issue - surely somewhat important, something that must be considered in the whole. But it's not about the software itself, how it works and what value it delivers.
I think Vinnie rightfully points to the fact that big companies still may choose the in-house solution due to a need to customise while the small can live with generalised solutions.
The small firms having so many ways to do things differently than their competitors, if their CRM or whatever process is identical to the next competitor it would not matter. And generalised web based stuff is really, really easy to try out and implement so I can see the lure.
But when the SMEs mature and grow they must distance themselves from the others, become better and different even in the small or core processes, and then customising comes in as a must.
Today that seems to require in-house systems and much implementation and maintenance costs.
I would agree with Vinnie that the real value will happen when the fledging SaaS transforms itself into SaaCS - Software as a Customisable Service.