My friend Sky wrote an HTML/CSS debugger for iOS - http://skyapp.co.uk/dbug/ - it doesn't give you all the functions of Firebug, but it's very handy and easier to start than this.
I find that majority of iPad issues can be solved by just resizing safari to the iPad dimensions. Most issues I come across appear on both platforms so troubleshooting isn't too bad.
It works well, except when you have to deal with a really mobile specific stuff (e.g. -webkit-min-device-pixel-ratio, -webkit-text-size-adjust) then you're left with no choice but to debug it on actual device (or in a simulator) although using Firebug on a small screen device is really awkward.
WebKit Remote Debugging[1] would be an ideal solution when it lands. For the time being, weinre[1] looks like a good (stopgap?) solution.