What are the penalties(if any) from search engines for pointing
multiple domains to one main site.

Example: I own and have build a website out of petwidgets.com. I then
buy the domains catwidgets.com, dogwidgets.com and birdwidgets.com and
point each of them at petwidgets.com, but never put any content
directly onto those 3 extra domains.

Is this considered duplicate content or spam? Will this in any way
effect a site's rank or position in the SERPs?

Could you be more specific to what you mean by redirect?
Are you pointing the host DNS records to the same IP address? or
Are you hosting multiple dummy websites that are redirecting (Error
301, etc.) to the main site on another IP address?

Although I did not set any of these up myself, I believe it is being
done with 301 redirects. 301 is what I get when I check the server
header on any of these domains. No sites ever existed at these
domains, so it's not like we're buying high-PR expired domains and
pointing them at our site. More like just grabbing good
industry-relevant domains before someone else does.

There is a way to make your sub sites redirect without any peneltys
from search engines.

Step 1: META all subsites HTML with the discription and keywords of
your main site you are redirecting to.

Step 2: Use this code 
"<meta http-equiv="refresh" content="2; 
url=http://yoursitedomain.com">"
to redirect to your main site. Change http://yoursitedomain.com and
the contenet="2 <-- seconds until redirects.

Hope this helps!

I forgot to add, you must have a index.html on your sub sites. If your
hosting provider does not allow you to have multiple addon domains,
you might think about switching to www.ClutchHost.com Our prices are
cheaper and we allow up to 9 addon domains with their own /directory!

As far as I am aware, if you are using a standard redirect (DNS or via
the Plesk control panel at hosting providers such as
http://www.netspaceonline.net) and not adding a doorway page which is
kind of like a index.html page which is plastered with keywords to
allow you to gain higher search engine exposure for those keywords you
should not have a problem.

If you are still in doubt, the best advice would be to contact the top
search engines directly, Google, Yahoo, MSN etc, I am sure they will
be more than happy to assist you directly with your query.