Via a new short little video uploaded to the simonscat Youtube channel yesterday (it was also mentioned on the Tandem blog), I found out that they are issuing a special t-shirt in a limited edition run of only 500.
The shirts are organic cotton and are on sale at http://www.simonscat.com. The proceeds are going to the RSPCA.
Life Begins at 41...or maybe 43 
I see that the little video clip is now unavailable (well, it wasn’t really a video but you know what I mean) and there is a note on the site that all of the adult shirts and most of the children’s shirts are gone. Awesome! There are also hints that a fourth cartoon coming soon: “Let’s just say, Christmas may come early this year…”
My shirt arrived yesterday. It’s quite nice. Love the picture on the address label.