Sadly, its not supported and Numbers for iPad strips out all the conditional formatting. If you edit the spreadsheet and send it back to your Mac Numbers or Excel, you'll have lost all the conditional formatting.
As a side note, I've found the performance on conditional formatting even on my Mac to be terrible. I can't even type consistently into cells without it missing keystrokes when there are complex rules in place. Until they fix the performance, there is no way they'll get it working on an iPad.
I really wish they would just ignore the conditional formatting on the iPad, but at least not strip it out altogether.