However, there are a few hitches. Because the information you need lies on the Armory, you'll have to enter a Battle.net password into your calendar app to access that (which as Bob says, should be fine, but if you don't trust it, don't do it). Also, Blizzard is apparently not that happy with people accessing this data on the Armory either -- they've removed links to scripts like this off of the forums before. It's probably not a bannable offense (they probably just don't want tons of calendar queries into the Armory's servers), but if you try to connect this stuff together, you'll do it at your own risk.
Still, it's a shame Blizzard isn't more open with this guild data -- it would be extremely useful to easily export guild information out to an .ICS file or even an RSS feed (for bank and join/quit info). We're not sure what all of their concerns are, but given that the info is already updated on the Armory, there has to be a way to more easily get it out of there and into more places where we can use it.