I don't have an iPhone at the moment, but this seems to have popped up in iOS4 when there is a connection problem amidst downloading an email. My sister comes to me with all of her iPhone issues. For her, I just closed the mail app (from the multitasking dock.) and when I reloaded it the email "rechecked" itself.
iMac 27" - i7/2TB/8GB
iPad - 16GB (wifi only)
iPhone 4 - 32GB/Black/Telus
MacBook - 2.16GHz/160GB/2GB/Black