I called CIC this morning as I asked about this same thing and they stated that they can not guarantee re-entry back into Canada. Just like any other work permit. Because it's a BOWP your intention is to work in Canada so they prefer you to stay while it is being processed. If you are applying for PR etc then this could be a red flag if you are denied entry.
Given my experience, you'd have no problem re-entering Canada. As far as you can provide evidence that you have applied for work permit (such as a receipt of BOWP), they'll let you in.