How long ago have you submitted your application? Try calling your prospective employer so s/he could send you a copy of your contract. CIC probably wants to ascertain the employer is still interested or still needs your services.
I submitted my application last November 2008, then they scheduled me for interview this January 28, 2010 only. Yes, most probably High Commission of Canada wants to know if my employer is still interested for my service. But the problem is, I have sent emails to my employer lots of time asking for the latest copy of a letter stating a job offer for me, until now I got no reply from him. It seems that he is no longer interested of hiring me.
What comes on my mind now is, I will still come for the scheduled interview but I will tell the officer that my employer is no longer interested of hiring me. What do you think?