(Long comment here. Hear me out :)
When you see a Big Corp logo on a small-ish SaaS company site, it's not the whole company using it. Just a small team.
Even when a plan says "unlimited," I can promise you multiple people in the same company are buying overlapping services. Why? Because folks want their own username/password/account management. (Plus they probably aren't even aware somebody else in their giant company is using it. I can pretty much guarantee they aren't IT-driven purchases. Not that they shouldn't be.)
As somebody else pointed out, employees can buy pretty much anything up to $1000 without approval from a higher up.
When things truly are being used by an entire large company, (driven by IT) they usually have some sort of account management/identity system that can hook into the company's own system (almost always Active Directory). Things like Google Apps or Salesforce, where you really don't want overlap (because you want to manage groups and permissions) have this capability AND don't have tend to have unlimited plans. (Unless specifically negotiated, you pay per seat.)
(In the future, OpenID or whatever evolves from its principles may replace it. But in the enterprise, SAML/AD/LDAP is kinda it for now.)
SaaS offerings that talk to these systems and are sold to Fortune 5000 corps typically start at $250,000. Somebody much smarter than I has outlined this strange enterprise price gap ( < $1000 or > $250,000). It was possibly Joel Spolsky. It's not a crazy theory, it's real. (As somebody else in the thread pointed out, anything above $1000 requires approval and lawyers, and is a really expensive for you as a vendor to deal with this for anything less than $250,000).
Tangent:
If you're a SaaS developer and want to sneak into the enterprise, get into Google Apps Marketplace. If you hook your app into Google Apps, any company already using Google Apps can use your app with a little less work (for them) than if you were just a regular web app.
As of summer 2010, there are about 30 Million Google Apps "premiere"(paying or enhanced) users.