I think there are a few things going on here which limit...

I think there are a few things going on here which limit arbitrage opportunities
Firstly I was only referring to institutional traders who can work on both markets simultaneously. These are the guys with all the tools and contacts to do it effectively.
Secondly there needs to be absolute liquidity otherwise the spreads make arbitrage a lot more difficult or a lot less profitable.
I'm getting the impression the US market is not all that liquid in this stock. This will change when MGC get a few more runs on the board, complete the Varmco deal, stop paying themselves in salaries and stocks for "performance" and start earning some honest revenue.