The SmoothSpan Blog has a very detailed article outlining the SaaS universe and the likelihood of dominant players taking on aggregation roles.
This is a segment ripe for consolidation--or maybe aggregation as users realize they are locked into individual SaaS apps.
OpSource acquired billing provider LeCayla earlier this month and odds are that won't be their last purchase. OpSource is in a unique position of having critical mass in the SaaS hosting space and there will be lots of companies that they see value in or can pick up if the company can't go it alone.
Taking the scale of Salesforce.com's user base out of the equation and the OpSource ecosystem is much more appealing, if for no other reason than the fact that OpSource can absorb acquried companies much quicker (thanks to their architecture that puts all the customers onto the Bus) something that Salesforce.com can't do easily.