Since FYI stands for "for your information", I would tend to use it like this:
FYI, I think the fish has gone bad.
In other words, the acronym simply replaces the phrase. However, I've heard FYI used many times in the following way:
As an FYI, you shouldn't mention his ex-wife.
Here, it is being used as a thing. This doesn't quite sound right if you fill in what the acronym stands for. It's as if a for-your-information is taken to be an object in a similar fashion to a(n) FAQ.
Anyone have some insight as to whether the second usage above is considered valid?