The Anointing of Jesus - One Event or Two?

If you've read through more than one of the Gospels, you'll have noted the accounts of Jesus being anointed with an expensive perfume by a woman.

Are these accounts describing two different events or are they all describing a single event using different details?

This article in White Horse Inn and the accompanying chart (which is really a great way to look at this) lays out various aspects of each description and points to this all being the same event.