The guys at Joyent are some of the smartest in the business (and not just because they're currently the biggest host offering Virtualmin GPL and Virtualmin Professional). Their support guys know quite a lot more about modern web apps than most hosts. Solaris Zones, which Joyent uses for their accelerators, are also among the most stable virtualized systems available. Xen is also very good, but if I were building a large-scale virtualized environment today I'd probably use Solaris Zones for it.
hmm i have a joyent accelerator. It is rock solid, except trying to get a recent version of python is a royal pain (python 2.3 comes installed, you have to wrangle a bit with pkgsrc to get python 2.4..).
If you use python, and don't mind burning hours trying to just get your environment up, but really want solaris and their nas storage (which is really nice), this is a good option. However, there are alot of really cheap ubuntu/xen based vps's out there that are Really affordable and ready to take on your application in its early days. From experience i would recommend that for python developers.
