jobs have different notice periods, mine is a month, i've been there 2 and a bit years.
a new job which i've just got (dont know if i will take it..) has 1 week notice for the probationary period of 3months, after that its a month.
Your employer can suspend you instead of you actually working your notice, usually with pay! It depends on your contract really.
I work in finance and theres some points in the contract about not working for another financial company within 10miles of the head office, or about trying to entice staff\customers away or leaking procedures etc to the competition but i think most of those points are due to law and regulations in that sector... so i wouldnt want to mess with those (some harsh fines & penalties for stuff like that..)
Put it this way:
I worked for Game for 3 days and left, and i've never disclosed that on a CV to any employers, i also worked in Clarks for a week, then walked, and have never disclosed that either!
My mum walked out of a job after 3 weeks, and straight into another one, they just opted not to give her a reference and the new employers werent bothered.
:D
i spose they COULD enforce the contract, or threaten too, but they probably wont due to MONEY and TIME, which is in addition to getting a replacement for you etc etc.. these are 2 things companies dont like wasting.... what they gonna do, MAKE you work for them? :P