Are we talking about an 07 graduate? I will assume yes with my comments below.
I assume "preferred" walkon is the same as "invited" walkon. Usually that would mean the player is pretty much guaranteed a spot on the team...i.e. no tryout necessary. And sometimes it even means a decent spot on the team (i.e. not in the bottom 10).
In any case, it seems to me to be pretty early in the recruiting season (for 07s) for a school to be out of scholarship money. Is it a fully funded program (i.e. 11.7 scholarships per year)? But then again, players are committing earlier and earlier in the Junior year, so I suppose its possible. But I'm skeptical that they're really out of money already.
And lastly, I don't think there's any need to formally 'commit' to a program without any scholarship...most certainly not at this point in time. You can state your intention...but I wouldn't 'commit' with my word without money.