Hi Dave,
>> What decisions have you made about scalability?
I started using GrapheneDB.com. Having some neo4j specialists watching over your DB really helps while you are building your application.
As soon as this balance shifts towards "in-house is cheaper" mark - I'll start hosting neo in-house.
This is directly related to scalability: you can setup a EE cluster yourself - no problem (that's what I did initially). However, you'll need to constantly monitor this, do backups, etc. etc. That. And also, by the time you will really feel the need to scale, you'll have a lot of data. So this should mean you already earn enough to have good choices (maybe host internally and have a dedicated neo specialist).
>> But I wonder if eventually hosting on EC2 means I have to go the Ubuntu route. Do you have any suggestions?
With EC2 actually you can use almost any OS/flavor. For example I was on CentOS for some time but for my project decided to move forward with Debian. Maybe something else would work for you better. Cannot give any suggestion here as I have no experience running neo4j for a long time to know its OS-related specifics, if any.
D.