I know ... It's a complicated situation. I was *supposed* to be with another advisor, but his co-PI got sick and they're not renewing grants, so that fell through. He sent me to this other advisor (who's the one I actually really really want to work with anyway), but she's on sabbatical next year.
In terms of everything besides funding, I've heard that having your advisor on sabbatical the first year is actually not too bad because I'll be taking mostly classes anyway. It's just whether they can provide me with another assistantship for that year, I guess.