The SimplyGames (UK) workaround not only sounds sketchy, but also unfair to Aspyr, who ported the game to Mac. I think we should support Mac gaming by supporting Mac developers.
If you're interested in a more "legitimate" deal, try the MacUpdate sale for $44.99.
http://www.macupdate.com/app/mac/46067/borderlands-2
It isn't a workaround - it is a shoparound. It is a perfectly legit purchase.
Those who don't shop around deserve to pay RRP.