http://www.wdwinfo.com/news/General_Disney_News/Celebration_buttons_for_Disney_Vacation_Club_members.htm
You'll be able to get them from Guest Relations or your DVC resort like any other celebration button. It also sounds like the DVC information booths will have them as well.
I have to admit to being torn on this one. I loved wearing our "Happy Anniversary" buttons this last January, because we were actually celebrating something (our anniversary - go figure). We were wished "Happy Anniversary" by so many people it was amazing and it really put us in a celebratory mood.
But being a DVC member isn't something I really "celebrate" the same way. I'm happy to be one but that's because I love going to WDW and staying in villas when we go. I'm not sure being wished "Welcome Home" in the parks is going to do anything for me. I like hearing it at the resorts where we stay, but I think hearing it at the parks is going to be a little weird. I'll think about it more, but I just don't put it in the celebration category. Other PP's are probably right, this is a thinly veiled advertising campaign.
That having been said, I don't personally mind advertising for them in that way. We really like our DVC membership and we already tell others about it. If were happy with anything we own, we tell others about it, so I don't really see a difference here. Of course, I respect that others won't feel the same way (either being happy enough to advertise or willing to advertise this way on principle).