Consider in the time and effort that you may have to put into silkscreening pens on your own, you may want to search the net for companies that sell those things in large, cheap quantities.
These guys sell all kinds of pens, including recordable pens:
http://www.coolpromoproducts.com/index.php3
These guys have pens with your print for as cheap as 15 cents a piece:
http://corporatelogos.logomall.com/default.aspx
Just to a search for "trade show giveaway"
(my two cents)