It looks like the biggest leak of the Play Store's soon-to-launch gift cards may be coming from Google itself. A support page showed up in search indices (that has now been pulled) which confirms the cards will be US-only at launch and will come in $10, $15, $25, and $50 increments. Through some Google-fu, we've also learned your Google Play balance will have a $2000 limit and cannot be used on subscriptions or devices. So, sorry about your plans to buy a hundred $50 gift cards and then buy 25 Nexus 7s. That's forbidden.
Below is what we've been able to piece together of the currently-unavailable page.