Comparative advertising without the comparison?

Slate.com's RSS feed is behaving rather badly. I therefore saw Slate's homepage, which has this ad:
The banner reads "Don't give up on Vista." Or at least it does at first; I saw it and thought that was an interesting ad tactic by Microsoft, especially on a site that used to be owned by Microsoft.

As it turns out, however, the ad is not for Microsoft at all, as the presence of the Mac guys on the lower right (which I initially missed) indicates. The ad lights up: "Don't give up on Vista." It's cute, as the Mac ads are. But I wonder how many people thought it was a bad Microsoft ad, not a good Apple ad.