I had a nice note from Roku support:
Regarding the question on whether we can have Hulu streaming on two devices at the same time, the answer is no.
Hulu Plus is available on one device at a time. If folks have been able to play it on two devices at a time, don't expect to continue doing so in the future.
You can find the specific details regarding your subscription to Hulu Plus at any time by clicking on Your Account. You agree that your Hulu Plus account is for individual use only and your Hulu Plus account is limited to only one simultaneous stream at a time. Additionally, you agree that for various reasons, such as restrictions from content licensors and other limitations or considerations from third parties, certain Content that may be accessible through one Property may not be accessible through other Properties.
You are right that it may be locked down at anytime (I've been saying that for months about Netflix), but right now, it still works for me. I just tried streaming on 3 devices and it wouldn't allow streaming to the 3rd device (which is supposedly exactly how the Netflix $8.99 plan works).