I just arrived, and just realised I didn't write my passport number on the the tm 6 form.  I just forgot somehow.  Arrival and departure have no passport number.  The immigration officer didn't say anything.  What should I do?  Is it a big deal?  I assume the form and passport are both on computer together.  

It should not be a problem. The number of it and your passport number are entered in immigration's data base. Just put your passport number on the departure card.

From past Thailand News reports, Immigration say the TM 6 form is redundant and not used by them, and even mentioned getting rid of it. But the Office of Tourism says the form is still necessary to them.

 

The stamp and dates in your passport are the really important bits. Just make sure those are correct.

I'm actually at the airport still, and I just talked to the immigration supervisor.  He was very nice and friendly, and said that it's OK.  The form and passport are linked together on computer, so it's fine.  I just need to make sure i write it on the departure card.