Can my iPhone have more than one iCloud account on it at a time?
No - the iCloud section of the Settings app only allows for one account on iOS 5.

You can however add as many iCloud accounts to the Mail, Contacts, Calendars as you have time to set up. You will see that you can enable the majority of the services on each subsequent non-main iCloud account.

The following items work only on the main account:


Backup
Documents &amp; Data
Photo Stream


The following item is exclusive - any one (but only one) account can have:


Find My iPhone


For example, I use one iCloud account as my primary personal account and have a second account for a consulting business. Here is what that non-main account looks like when set up.