Yes you have unlimited sms available on any SERO plan. At least so the website says, I have heard some have probelms with this, but Im sure you could point the rep to the website and they would remove charges if any was made. Also so I have read using the SMS hack you can send text messages as a sms without attaching a picture and you wont be charged! But you will be charged definitly to recieve.
~Mike
EDIT: I'm not a 100% sure about not being charged, because I also have the SERO plan. But the reason behide it is because in sprints eyes this phone isn't pic/vid compatible.