What are the consequences of jailbreaking an iOS device as a Developer?
Consequences: 


increased personal responsibility to not install software or firmware that might prevent restoration to a supported configuration.
increased personal responsibility for device instability or security vulnerabilities
less software support from the vendor (may be moot if you run beta software as there is less support for that as well)
potential delay if you ask for a hardware repair (either within or without the warranty period) if you do something that prevents a factory restore I suppose you might be out of luck, but I consider that further action (or error) past a jailbreak. 
reduced confidence that the app will be of use in debugging issues 
having to determine if an issue is in all devices or just in jailbroken devices (such as frequent and intermittent issues with push notification service).


I have had hardware exchanged by Apple when running beta software - the technician was able to tell it was clearly a hardware error and I was open with her when I presented the device for service. I wasn't made to feel like I had done anything wrong. You could be denied service if you're running software not designed by the manufacturer, I think a little common sense will let you know if it's worth the risk.

I see jailbreaking as taking down a guardrail. If you crash and the "authorities" rule you would have been safe had you left up the protection, you've got less grounds to stand on. From the comments on another answer - a several people disagree with this (and that's better than fine, many viewpoints is a strength here)

On the plus side, if you know what you are doing, you might be able to patch known vulnerabilities better than or faster than Apple does. You might also gain knowledge you would not in the process of jailbreaking and dealing with the device while the guardrail is down.

But to gloss over the difference between hardware support and software support or just stating "jailbreak=bad" over simplifies a complex issue in my eyes.