Access your Zotero library on your iPad; opens PDFs in a PDF app such as PDFExpert and enables you to annotate the article then send it back to Zotero. 2/9/2012: See this valuable update
from ProfHacker from August 2012 showing how to use ZotPad. 10/15 update: ZotPad does not seem to be being updated; users may prefer PaperShip.
Note: once annotated and sent back to ZotPad, the version you see on Zotpad will not show the annotation, but they will be on your browser/desktop/online Zotero libraries (and if you open the PDF in the app you annotated it in, you will see your annotations). Some annotations (i.e. text box content) will not show up on your Zotero.org online library - you will see an annotation icon, but not actually see the text.