Don't get hung up on the 90 day thing. Even if you pass the probation period you're not obligated to stay with them (unless you sign a contract). Usually you have updates on how you're doing from them every 30 days during the probation period (or more often). During the probation period at my last job neither the employer or I realized the probation period had passed.
I've been working as an RN for many moons and every one of the places I've worked had a 90 day probation period.
I like the probation period because if you don't like the situation you can leave with no hassle. On the other hand if they don't like you, the same (I believe they say "You're not a good fit for us". Don't take it personally; it's just business.
Concur...don't burn any bridges; Murphy's law will get ya every time.