Are there any services offering a dollar cost averaging approach to Bitcoin investing, i.e. something like an index tracker fund which purchases a set amount on a regular basis? If not, are there any reasons why such a service can't exist:

• Are there any theoretical disadvantages, i.e. anything specific about the asset which discourages this style of investing?
• Are there technical limitations, e.g. is there some restriction imposed by payment bodies which would prevent making a regular payment from Paypal to Dwolla, from Dwolla to an exchange, then buying coins, in an automated fashion?