The sump is quite exposed, particularly if the shields are missing, and so corrosion can easily occur. It's uncommon for it to get so bad that it leaks however - I'd think twice about taking your car back to whoever services it - this should've been spotted a long time ago.
Replacement is relatively simple given that it's only bolted on.
Needless to say you need to get this done some time between yesterday and now as you really do not want the sump casing giving way completely dumping the only thing between you and a seized engine out on to the road...
Mathew