Doh! That worked.
I had been making the portal too big, apparently, and had been building it on a "stand". I started MC in 1.7x, which is more forgiving when it comes to Nether Portal design, I guess.
Here's what screwed me up, I think:
O=Obsidian
S=Stone
G=Ground
OOOOO
O O
O O
O O
OOOOO
SSS
GGGGGGGGGG
Thanks!