Let me just get this off my chest.  Can we stop using the "i" or "HD" thing for titles of programs.  It's getting annoying and old.  Now that's been said, check out this pretty neat program called iSculptorHD.  It's only for the ipad, but they do have iSculptor for the iPhone.  It doesn't allow for  a lot of polys, but it is great for speed modeling and those with a fix of modeling on the go.  If you want you can work on iSculptorHD for pelimenary modeling than export the 3D object to a preferred 3D package to tweak and add more details.  Visit itunes.com to grab the app.  Check out their support page HERE. Here is the iPhone version.  Got to say, it is pretty smooth.