I phone 4 and Imac Icloud text issue
If their name at the top of the message window shows in red, 'apple' thinks it's an sms-only number. 

Check &amp; if necessary re-select their proper connection details from that red name
[becomes a drop-menu if you click it - down arrow to the right of the name]
Will turn blue when it connects successfully, or stay red if it is only SMS-capable.