Nope. No potions can be used in bear form.
Form shifting is instant though -- as in a full running jump, click bear form and you'll be a bear mid flight -- or cat for that matter...
Same with shifting back to human form so if you want to, simply shift to human, click back to bear. What's lost is accumulated rage.
Oh yeah, you also regen mana in bear form, you just don't see it regenerating.