Atlassian and JetBrains are competitors. Don't see it happening.

Though a Salesforce, Atlassian, and JetBrains three way merger would be cool.

Are you sure? JetBrains uses Atlassian http://confluence.jetbrains.com/dashboard.action

I stand corrected. They seem to have collaborated on other tools too.

But they seem to be in direct competition in this could make for nice integration with Teamcity and Bamboo is a competing product.

They do already have that tool, you know? https://www.atlassian.com/software/crucible/overview (but I think Upstream has every chance of blowing it out of the water)

Perhaps you meant Fisheye ? https://www.atlassian.com/software/fisheye/overview

This combines both of them, both source repository viewer (fisheye) and code review (crucible).