Not sure how many people have noticed or had this problem but if you exceed 4 sms as a single message a popup flashes up informing you that the message has been converted to an mms. It happens quite quickly.
After a lot of useless info from helpdesk at t-mobile ranging from software update will cure it to, 'I am sorry we do not have a phone called a Pulse', i got to speak to the technical dept.
To cut a long story short he assured me the Pulse is preset to convert 4 txt to an mms and this cannot be altered from within the settings. He suggested a 3rd party app like Handcent as a 'workaround'.
More info here: Problem with text messaging.
Hope this helps people not to run up a bigger bill than they expected.