How useless is that? So I've standardized on always accepting the first calendar in the list because at least that way I'll always know that any events I created on my iPhone are in that one.
Even worse, after I click OK it shows a quick animation of adding the event but then immediately another similar dialog for the next event before I have a chance to see where the first event went.
Finally after all events are imported in this one-by-one fashion with the useless "which calendar" question, I get an alert box saying all events were imported successfully.
A principle of user interfaces is don't ask the user a question if you're not going to provide any information for them to make a decision. If you can't provide the facts that you want the user to consider when making a decision then just go ahead and make a sensible guess yourself because the user won't know what to do.
But in this case, iCal does have plenty of useful facts - the event information - and isn't showing them. And more than that, iCal knows what calendar that event came from on my iPhone (or it should, unless they really botched up their internals). So if there is an identically-named event on my iMac, that would be a good first guess, and I'd prefer if iCal just did that and didn't bother me.