That happened to me. It was awesome. I was 'naving away and then my girlfriend called. So I'm talking to her and suddenly I hear "in 800 feet, turn left on such and such drive". I was like "omg, did you hear that?" and she was like "ummm, no, are you an idiot or what?" and I was like, "that voice, there was a robot chick telling me things just now" and she was like "hmmkay". And then a couple minutes later she was talking and I heard it again and I was like "Listen! Listen. Do you hear her"? And she was like "Her?!" Who the F are you talking to? And I was like, the chick in the navigator thingy.... Ha ha.
On a more serious note, I like that an incoming call doesn't over ride your directions. I would hate to get lost just because someone called in the middle of a trip. Also, wouldn't it suck to not be able to answer the phone just because your navigating? Thats how my Dare was, you could not do both. I love my Droid!