Yes, I have the same problem with an iMac and a secondary monitor. The problem occurs with Chrome, Firefox, and Safari.
It certainly seems to be a problem with the Sling plug-in, because I can do the same thing with youtube videos, and they will properly go into presentation mode on the secondary monitor.
As a practical matter, presentation mode greys out the alternative monitor, which is not exactly good for multi-tasking. But simply expanding the browser to full screen with the green button, as opposed to the dual arrows in the upper right, works on the correct display, so that is how I plan to continue using it.
I forgot about double clicking the screen, as opposed to using presentation mode. Dragging the menu bar to the secondary monitor is an interesting workaround. Not ideal, but after trying it, I see that had two problems 1) menu bar becomes transparent and remains visible (it does not do this when the menu bar is on the main display and the browser is there) and 2) it is zoomed such that most of the content is off-screen.
So without changing the menu bar, double clicking the browser plugin, the video jumps to the primary monitor, no matter where which monitor the browser resides.
When you double click a youtube video, it always expands to full screen correctly on the same monitor on which it resides.
It seems to me that a simple change to the plug-in code would fix this.
It would appear this worked differently for the old player though? I.e. it was more logical in that whatever display the browser window was on would be the one to go full screen.
This was ideal as I tend to connect my MacBook to a Cinema Display, and do most of my work on the big display (thus needing the dock etc on this main screen). It was convenient to then run the web player full screen on the macbook's native display.
What was the logic behind changing this? It Seems like a step backwards.
On a more general note, I preferred the old player because:
1) the directory was far more user friendly with big boxes for each Slingbox
2) it showed the streaming speed without havig to hover over the icon
3) the directory also showed Slingboxes which were set up with a direct IP address (this is still possible via the Windows desktop player software, even if you've prevented it via the website)
Don't want to diss the devs (I cone from a dev background, but games, and I know how hard it is to satisy everybody) but my personal view is that the watch site was just fine - why not put resources into improving the connected devices, or updating Slingcatcher to work with the new boxes - this would show real care for some of your loyal customers. Some of us are feeling a bit letdown at the moment.