I think at some point the freemium model needs to be abandoned. Dropbox is getting there - 25 million users (or accounts) and a service that works pretty well. At this point why not charge a nominal fee for the low end of the service - even if it's 12 dollars a year (only $1 a month!). Or a few months free followed by a monthly fee.
I'd be willing to pay after having used the service free for a while - although I personally will have to cause I need more storage.
You are assuming the high end of conversion at 5%. And you are also assuming those 25,000,000 accounts are all held by different people, when in reality there are people who hold multiple accounts under different email addresses for different purposes or to avoid upgrading. It's not so easy to switch from one acct to another, probably a good thing on the part of DropBox, but people still do it.
Regardless, it's a great service and if it isn't already a great business I'm sure it will be.
