A follow-up on this.
I too had $28 worth of this promotion expire. However, after having a conversation over drinks w/ an attorney and doing some research they may be doing this illegally.
If you look at the store credit history in your account it will show two separate account credits, one for $25 (paid) and the other for $45 (promotion). The $25 amount states no expiration date, (N/A) while the $45 amount (in my case) shows an expiration of 3/9/2013.
After reviewing their Legal Agreement as it applies to store credits, section F states:
If you have more than one in-store credit, then the credits will be processed according to the age of the credit, with the oldest in-store credit being applied first.
There is no other verbiage applicable to store credits sharing the same age, or if it will be at the discretion of GoDaddy to make this determination.
Ultimately there is a hole in the LA that may be misappropriating the credits and their legal obligation to the value of the collected funds.