Here’s the solution that is least likely to bite you in the ass:
Don’t replace the ipod. Buy her a new one and explain that it is a gift for a good employee, not a replacement for the one that was stolen. You don’t want to put yourself in that position because eventually someone else will get something else stolen and they will expect you to replace it. You probably won’t and that will leave someone feeling like you’re playing favorites.
It’s not a precedent you want to set as an employer. Just as important is not making your employees think you’re a callous jerk. Buy her an ipod if you really feel guilty and want it replaced, but make it very clear that it is a gift from a friend, not a replacement from an employer.
The way you present it can make all the difference. If done right you are explaining that despite the fact that you are under no legal obligation to replace it (and have no existing policy that would demand it), you don’t want her to be unhappy and her happiness as an employee is worth $150 or so to you.
Keep in mind that under the circumstances (unless there are egregious ommisions of standard safety practice that you haven’t mentioned) you are under absolutely no obligation to replace it. Irrespective of some commenters vaguely suggesting you could be in trouble because the workplace was not safe enough or because precautions could have been taken that weren’t, fear not. There is no precedent in US history where an employer was held accountable because their employee got robbed. You would have to be all but intentionally making it an unsafe work place before anything like that would stick in court.
Whatever you do, don’t simply replace it without explaining the whole “I’m just being nice here” thing or you could get in legal trouble in the future if another employees property gets stolen (or the same employees property gets stolen again). Inconsistency in things like that can lead to claims or racism, sexism, favoritism, etc. and those are things that can bite you hard in court.