How did I miss this? There's now a third-party iPad app for Zotero called ZotPad. I've only had it for a couple of hours, but here are my initial impressions.
For six dollars, you get an app that allows you to connect to your Zotero account, browse your personal and group libraries, view (but not edit) item details, and view or save attachments. Extremely handy! Frankly, I think it's worth the price just to have access to all my PDF attachments.
It doesn't allow you to write to your libraries in any way, so no creating collections or items, adding notes, or joining groups. It also doesn't have a way to create citations or bibliographies if you're writing on your iPad -- it's strictly a viewer. The developer is looking into adding improved search, support for viewing notes, and editing library items.
ZotPad requires iOS 5. I had to update my iPad in order to install it, which I'd been putting off for a while. I think it was worth the hassle.
I'm slowing down from fall semester madness just long enough to mention that there are now not one, but two Zotero-related apps available for sale in the Android store:
Zandy, a client allowing library access/viewing/editing from your mobile device, and
Scanner for Zotero, an app that replicates some of the Zotero "magic wand" function by allowing you to scan ISBN barcodes to add items to your Zotero library.
I've tried Zandy just a bit and it seems to work fine for what I've done with it so far -- I haven't yet tried out Scanner, but both apps look promising and interesting. (Obviously, I'm not affiliated with either project.)
Edited to add: iPhone/Zotero users should take a look at BibUp, a project of the University of Fribourg.