Yeah, Heroku should put up or shut up until they start offering alternate NoSQL databases (and no, MongoHQ is not an alternative because of network latency).
As they are hosted on EC2 couldn't you use Amazon Cloudfront with them without too much trouble and with no latency issues? http://aws.amazon.com/cloudfront/
The main issue would be integrating Cloudfront with Rails not getting it to run on Heroku. (A quick search doesn't show much support for using Cloudfront with Rails in general)
If Heroku offers their own NoSQL alternative, they will put it on a different EC2 instance and have the same network latency so MongoHQ seems like an equal solution. They provide private IP addresses to connect to so it is the same as running it on your own separate EC2 instance. How does anyone separate their app and db server without creating latency in your opinion?
Interesting article though.