Yep. I have heating pads underneath my hedgie cages too. They're great for a little extra boost after bath time or right after I've been cleaning the cage and changed the liners.
In short, they're great for supplemental heat, but you don't want them as your hedgie's primary heat source.
For one, the human version (I don't know about reptile to comment validly) heating pads have auto-shutoffs for safety purposes. The other part is that they don't cover the entire floor - which is kind of good and kind of bad... You can't get an exact temperature with them, so if they're too hot hedgie is limited to walking around the outskirts. If s/he falls asleep on it, s/he can end up with a burn. If the temp is just right, then hedgie is fine as long as s/he stays on them, but can get chilly if s/he falls asleep to the side of where they are. Also, things above (like the wheel) may not be sufficiently warm for hedgie.