When it comes to personal prayer, I find it annoying. If you are talking to me, then don't offer. Offering to pray only tells me, "This person hasn't paid enough attention to see that I won't like this." However good the intention may be, it is overwhelmed by the demonstration that you haven't cared enough to pay attention. I might or might not say anything, but that is how I will feel.
I am far from alone in this group in this.
OTOH other people here, for instance tseliot and static, are Christians, and undoubtably would appreciate the gesture if an occasion called for it.
So offering to keep someone in your prayers isn't necessarily inappropriate, but it will be for some. If you aren't sure, then I'd suggest not doing it. I'd strongly not suggest doing it with me, Christian, Scott...
Cheers,
Ben