I have seen a couple commercial IPAs that used Lactose, the only one I've actually tried is Apocalypse Cow from Three Floyds. It's a really good beer, but it is quite sweet for a DIPA. I guess it comes down to your preference in how you like your IPAs. I prefer my IPAs to be as dry as possible. This is just my opinion, so if you like a little sweetness in your IPA and you're feeling a little experimental, I'd say go for it. It's nothing that hasn't been done before. If it were my brew, I'd probably save that lactose for a milk stout or something like that.