I don't think nada is the best choice. My 10 yr anniversary was about 10 days ago. We didn't talk about it in advance, but we exchanged cards. Mine to him was short and sweet, a blank card that I wrote a short note in (Have I told you about my idea for a line of greeting cards for separated/estranged spouses? There are NO appropriate cards out there for our situation!) His to me was a "Thinking of you" card, which he filled with a long note for me. More details on my thread.
I think you should at least get her a card and write something sweet, but not romantic in it. I took Puddle's advice and wrote that I was glad that he was part of my life. Did you ever have anniversaries when you were S before? (I think I remember that you've been S more than once, right? If I'm wrong, just ignore my babbling.) If you don't acknowledge the day in anyway, I think W could definitely hold it against you ("that bum forgot!"), but if you do something small, the worst she can say is "That fool is still in love with me," which she already knows.
