Merchandising really differs from con to con. In most parts, the merchendise will be con specific, as was most of the merchendise at Gatecon 2000.
Although you will find that Bryan from WOLF359 has some officially endorsed SG-1 merchandise available. I'm not sure of the full range, but I know that there are some pretty impressive T-Sirts there. I'm not quite sure, but I think that the WOLF official merchendise is limited to the UK only, and he is unable to sell outside the UK. Don't quote me on that, but i'm sure that is the case.
You will find at some cons, official merchandise sales. MGM had a stall set up at Gatecon 2000, and i'm sure they will be back for 2001.
There will also be new products available once MGM authorise new products.
Hope that sheds some light on the subject.