Also, is there documentation on exactly what data is stored on Pulumi's servers? There are cases where it's unavoidable to have sensitive, if not directly secret, data in cloud provisioning and I'd like assurances that it's not accessible.

There is not at this moment, but I agree we should do this. I will file an issue and we'll do this over the next week. We never store your creds, it's just inputs/outputs for resources.

pulumi stack export

will show you the precise data that is stored.

Yeah - the main concern is those inputs sometimes being A Little Sensitive, though I've gotten pretty decent at avoiding too much of that.

Are there plans to open source the Pulumi server?