Thank you Paul! Knowing the WHY behind something always solidifies my understanding of a concept. It does make me wonder why the cheat sheet linked by Sangeetac and Moz's Beginner's Guide to SEO both use that scenario as an example for a 301 redirect - it certainly adds to the confusion.
Alan - while we are on the subject...here is the code I used for the 301 redirect, which was my first ever using .htaccess. The problem is it redirects twice from certain starting points: first from site.com/default.asp to www.site.com/default.asp then to www.site.com/.
RewriteEngine On
RewriteCond %{THE_REQUEST} /index.html? [NC]
RewriteRule ^(./)?index.html?$ /$1 [R=301,L]
RewriteCond %{THE_REQUEST} /default.asp? [NC]
RewriteRule ^(./)?default.asp?$ /$1 [R=301,L]
RewriteCond %{HTTP_HOST} !^www..* [NC]
RewriteRule ^(.*) http://www.%{HTTP_HOST}/$1 [R=301,L]
Initially I tried this (which I think is what you are suggesting?), but if I remember correctly, it wasn't taking care of default.asp or index.html (don't quote me on that):
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www..* [NC]
RewriteRule ^(.*) http://www.%{HTTP_HOST}/$1 [R=301,L]
Thanks for helping me do things the right way!