Never use Apple gift cards
By now most anyone who follows Apple news will have heard about Paris Buttfield-Addison's saga, wherein redeeming a legitimately-purchased Apple gift card led directly to his Apple account being locked and his access to all his data — including a multi-terabyte photo library — revoked. You can read more about this here.
This was eventually resolved, thanks only to his prominence in the Apple blogosphere (is that still a word we use?) and the media shitstorm that eventually arose from this incident. You or I, with less substantial followings and fewer books about Apple software development in our names, would undoubtedly not be able to get such a response from Apple.
This isn't the first time we've heard about innocent gift card activity leading to an Apple account being permanently locked out, either.
This all leads to only one conclusion: if you value your Apple account — including your developer account, your iCloud data, and iTunes purchases — don't try to redeem gift cards, no matter where they came from. There's just too much risk, and there's effectively no recourse for normal people.