Here's Rabbi Michael Broyde's excellent treatment of the subject, in which he permits observance of Thanksgiving (according to many) but prohibits Halloween, out of our prohibition of imitating heathen-inspired rituals. This matches the practice I've seen observed (at both lay and rabbinic levels) in mainstream Orthodox communities wherever I've been in America. (And while I can't vouch for Canada, I'd think the US is the country that most secularizes Halloween.)
Handing out treats to kids who show up at the door is a different question (addressed by Rabbi Broyde as well); between rabbinic writings, what I've seen done in practice by the lay population, and what is recorded as practiced by twentieth-century rabbinic leaders such Rabbis Avraham Pam and Yaakov Kaminetsky, it's certainly defensible (if not outright appropriate) to do so.
Here's Rabbi Broyde:
Based on this, in order to justify candy collection on halloween, one would have to accepts the truthfulness of any of the following assertions:
1] Halloween celebrations have a secular origin.
2] The conduct of the individuals "celebrating Halloween" can be rationally explained independent of Halloween.
3] The pagan origins of Halloween or the Catholic response to it are so deeply hidden that they have disappeared, and the celebrations con be attributed to some secular source or reason.
4] The activities memorialized by Halloween are actually consistent with the Jewish tradition.
I believe that none of these statements are true.
Applying these halachic rules to Halloween leads to the conclusion that participation in Halloween celebrations -- which is what collecting candy is when one is wearing a costume -- is prohibited. Halloween, since it has its origins in a pagan practice, and lacks any overt rationale reason for its celebration other than its pagan origins or the Catholic response to it, is governed by the statement of Rabbi Isserless that such conduct is prohibited as its origins taint it. (76) One should not send one's children out to trick or treat on Halloween, or otherwise celebrate the holiday.
Appendix A: Collecting Candy on Halloween Harmless Pastime or Halachic Prohibition? http://www.tfdixie.com/special/thanksg.htm#A10