In reply to Tyler:
Could be wrong but I'd be surprised if it has been done yet this year, even with crampons - it's all pretty high altitude so takes a long time to clear of snow. The camptocamp website might give you an idea...
http://www.camptocamp.org/routes/57744/en/matterhorn-arete-du-lion
There's probably a webcam view (or two) from Cervinia which will show how much snow there is.
Unless very snow-free you will almost certainly need crampons / axe, even if descending the same way. If you're going 'over the top' and down the Hornli you will definitely need them.