There is much misinformation in this thread. There is no legal requirement to reside in Canada while your application is in process until that particular provision of Bill C24 comes into force (if it ever does) requiring you to sign an intent to reside in Canada affidavit for the period while your application is processing. This is not currently in effect and even if it were it wouldn't apply to people who applied before it came into effect.
However, leaving Canada after applying raises some serious practical and logistical issues. As has been pointed out, some CIC staff disregard or are just plain ignorant of the law and will give your application extra special attention if they become aware you aren't living in Canada. Moreover, how are you going to be informed of your test and/or oath date? CIC has an increasingly bad habit of sending out letters giving you less than a week of notice? What address will you ask CIC to forward these notices to? Keep in mind, proving a "suspicious" address after applying has been grounds for an RQ in the past. But the biggest issue is the possible loss of PR status before they are done processing your application. With non-routine applications taking three years or longer, you better be sure you can maintain PR status or come back in time.
They are really asking for trouble with this "intent to reside" clause, however. What about situations when an applicant's spouse is a Canadian citizen and they need to accompany their spouse abroad? Do the geniuses in Ottawa expect these applicants to live separate from their spouses just to keep up the illusion of intending to reside in Canada? Its really stupid. Its just political posturing with no policy value at all.