This would seem a problem that could be solved by a cloudish sort of solution, though -- have an iPad app which hands actual typesetting of a document off to a server somewhere, sending TeX sources, and receiving a PDF in return.
A simple starter implementation might require your TeX to be a single file, and could just run TeX in a mostly-readonly chroot jail (bearing in mind that TeX can shell out, read/write arbitrary files readable by the executing user, and so on), and serve up a PDF, all wrapped in a web service of some sort.
This wouldn't be terribly fast, but given TeX's performance, the actual typesetting is likely to be the bulk of the time spent, even considering the round trip.
A simple starter implementation might require your TeX to be a single file, and could just run TeX in a mostly-readonly chroot jail (bearing in mind that TeX can shell out, read/write arbitrary files readable by the executing user, and so on), and serve up a PDF, all wrapped in a web service of some sort.
This wouldn't be terribly fast, but given TeX's performance, the actual typesetting is likely to be the bulk of the time spent, even considering the round trip.