iOS 7 Messages app reverts from iMessage to SMS at random
iMessage is essentially an Apple operated Instant Messenger service deployed to compete with, or augment, SMS and MMS.

iOS will use iMessage to deliver a message if there is a data connection available to Apple's servers and both the sender and recipient are known by Apple to have iMessage configured for their respective addresses.

Most likely if a person you regularly iMessage with is now sometimes receiving as SMS it is due to intermittent data connection (ie you are mobile and data service is failing). Otherwise it could be an Apple iMessage server issue - seems to crop up regularly.

Furthermore in the last three weeks several of my friends and family have reported issues with iMessage so possibly there is something deeper and worse going on with iMessage servers.

cheers