As far as I know, all iPhone apps work on the iPad -- they are just lower resolution. (That is part of the "charm" of the iOS ecosystem.) Unless there really is an iPad/HD version of the knittinghelp app, which I'd love to have!
I don't like using iPhone only apps because of they are so small on the screen or full of pixels when I make them 2x the size. I have an iPod touch and tend to use those apps on there.
The knittinghelp app is an exception because it is easier to follow the videos on the larger screen, even at low resolution.
While I'm being honest, I use the iPhone Fisher-Price Big Foot Monster
app on the iPad too -- the kids prefer playing it there over the iPod even though I think it looks terrible.
Going back on topic, I only have two knitting apps, neither of them are HD/iPad optimized: KnittingHelp Video Reference
and Knit Counter Lite
I also have Goodreader
for reading PDFs, but I don't save patterns to my iPad. I open patterns online as I need them and read them in Safari.
is awesome for social media and blogs -- it turns them into a magazine format and is my favorite way for keeping up with the blogs I follow!