Android certainly has an advantage here, but a smattering of iOS apps, including the EHR in question, seem to licence speech-to-text and integrate it fairly seamlessly. Siri (before Apple acquired them) and Orchestra (a new to-do app) are good examples.
It's also worth mentioning that screenshots of internal builds of iOS 5 have a microphone on the keyboard, seeming to imply speech-to-text could be a surprise feature of iOS 5.
I also think you're underestimating the usefulness of a tablet form factor when physicians are sharing information with their patients. Harder to do that with a laptop.