You have to acknowledge the day somehow. Ignoring it will piss her off (my opinion). Definately DON'T send anything that says "Happy Anniversary" that will only remind her how UNHAPPY she is.
"Thinking of you" pretty much says that you realize what day it is, and yet it's pretty non-threatening.
Leave it at that. Don't get sappy. Don't make it long. VERY SIMPLE, just acknowledge the day. I would do a text message instead of the ecard. Ecards always play pretty music and get too sappy.

MY OPINION ONLY!

Me 36
Husband 35
D5
S2
separated:
10/29/07-present
Served divorce papers 1/22/09
"When the world gives out beneath your feet, it is time to learn how to fly."