Just to clarify-the voucher they give you is good for more than 24 hours. There is an expiration date on the back (I think right now it's Sept. 2009) and you can use it until that time. I, personally, have used more than one voucher at a time. If I am doing a PP CD I don't use my voucher-I save it for a time when I won't be getting the CD. I think I have about 6-10 saved up at this point.
Yes you can do it once a day, and once per card holder. If you and your spouse have separate accounts, they'll give you 2 vouchers.

(or you and your DM, etc.)
At this time you'll either see Chip and Dale or Goofy. Mickey and Goofy haven't been there for quite a while now. CMs suggest making a complaint at City Hall if we want to see more characters.
Yes, most CMs will let you use the voucher for any PP taken. However, last May, while they were training new PP people, they were very strict about it. I explained I didn't want
another Chip and Dale pix and the Manager said he would let me, this time. That was how I ended up with Hitchhiking Ghosts and Darth last May.
Here's what's on the back of the voucher:
DISNEY'S VISA FREE CHARACTER PHOTO VOUCHER. Redeem this voucher and your number photo card for ONE 5x7 Disney Friends Portrait at Greetings from California in Disney's California Adventure Park. PHOTOS ARE ONLY AVAILABLE AT DISNEY'S CALIFORNIA ADVENTURE PARK 24 HOURS FROM DATE PHOTO IS TAKEN.
VALID THRU 9/28/09.
Now while it does say the photos are only available for 24 hours, they are actually available for 30 days since they are on your PP card and those pictures will be there for 30 days (to download) and then available on the website an additional 30 days from download date. This I know for a fact since I have many M&G pix on my CDs from Sept/Oct last year as well as Dec/Jan. this year.