Tom,
You are quite right - there is no way currently to connect any dive computers to an iDevice. Each cable/device would need to be licensed/vetted by Apple (and of course software created too) and it's really just cost prohibitive for such a small market. I did not mean to give you this impression. (*See note below*)
What I meant to describe above is that you can do everything else about logging your dive on your iDevice using Dive Log while you are on the boat or shore. Then when you return to your laptop/desktop machine, you sync your logbook to the desktop, download your dive computer there, and then merge in that dive computer data into what you already logged. So, using some numbers might help. Using Dive Log you create your dives numbered 42&43 entering site/GPS location information, weighting, maybe tank info, take a picture of the site, what equipment you used, have your buddy sign etc etc. Then later on, you sync your logbook back to the laptop, download your dive computer (using whatever desktop software supports it), and then import/merge the dive computer information into your already logged dives #42 & 43. Only the dive computer information will be added and anything else you already logged for that dive will stay. Then sync your logbook back to your iDevice so it's now in both places again.
Janice
NOTE1: If a dive computer supports transferring it's logbook information via a *wireless* transport, like bluetooth or wifi, then the whole licensing thing would not apply. Only if you try to plug something into the dock connector do you run into this.
NOTE2: There is now a generic RS232 (ish) serial cable available that is approved by Apple that you can plug into the dock connector. So if a dive computer had an RS232 serial interface, one could write software for an iDevice that downloaded it using this cable (and associated software library) with the dock connector on one side and the dive computer on the other.