In your posting you are describing two things:
+ the balloon message informs about minimizing to tray
+ the balloon itself makes the temperature visible for some seconds (if icons are hidden in system tray)
And both criteria are fulfilled by my second suggestion, aren't they?
The only difference from now is that the new balloon tip would give a one-off hint to the user (about minimizing to tray) so it would be permanently disabled after closing it.
Additinal to this, the new balloon could be provided with a [x] button. I this case the balloon tip would be permanently disabled only by closing it with the [x] in its right upper corner.
What do you think about it?
However, for me an additional option for disabling the balloon tip would be great, too.

Thanks again!
