Non issue, most companies use VMs for their dev environments for this reason among others.
At my company, we give developers several days (up to a week at times) to get everything downloaded, installed, setup, configured, etc. It's a huge drain but then again, you should've seen it 2-3 years ago, at which point the only avoidance of duplicated efforts was by virtue of having a single portable HD which held all of our databases [for use in testing]. The data was 5 years old at the time...
It's the first time I thought I could use a web-based editor. That's a big deal. Well done!
Best of all, unlike ST2, we're open source: https://github.com/ajaxorg/cloud9
However, the problem to me seems that in order to reap the benefits of Cloud9 one has to give up the benefits of their current IDE. It will undoubtedly be the same situation for LightTable when they release.
I want collaborative editing in my IDE. If I'm using Visual Studio and my colleague is using SublimeText, I want to be able to work collaboratively with him/her. I've wondered if some sort of protocol could be created by which other developers could create plugins for popular IDEs that might allow cross-IDE collaboration.