I do think it means that, unless you get other funding that completely covers everything in your budget, you will get the Wenner-Gren. My committee seems to think so, at any rate. One adviser said she'd never seen anyone get that email and not get funded, and a friend who got funded last year got the very same email. So, looks good. Congrats, Vilanelle!
For future readers, I thought I should post some more info so that next year anxious applicants can get a sense of when they'll hear back.
On Monday they sent out some rejections and Tuesday others who hadn't yet heard received an email asking what other funding they had received. Though this email wasn't an acceptance, it stated that at this point we had an "excellent chance" of being funded and shouldn't bother applying again for May. I assume they are trying to avoid funding people who are already funded. They said they'd give us a final answer in early May.