You may want to peruse the experience recently reported at the immigration.ca forum,
at this link.
Short version:
There is also a very recent Federal Court decision regarding CIC's approach to deeming applications abandoned and terminating them. See
the Karim Ayyad decision. What constitutes an acceptable reason for missing the scheduled oath is a key issue discussed in this case. (Applicant prevailed in the appeal, but it is CIC's position and approach that warrants careful attention.)
Isolated reports (even those reflected in official decisions) do not necessarily tell us how CIC is approaching most individuals in these situations, but it at least appears there is a significant risk that CIC is taking a draconian approach regarding foreign travel . . . that is, there is a risk that if the explanation or excuse is tied to foreign travel, CIC will find it is not an "acceptable" reason, deem the application abandoned, and terminate processing.
It may make a difference if the letter itself arrives at CIC
before the date of the oath. I think in the two instances referred to above, the explanation did not get to CIC until after the scheduled oath.