The present perfect in American English (AE).

Calliope   Tue Nov 07, 2006 8:47 pm GMT
Roughly: present perfect implies a "yet" or "ever" - Have you had breakfast (yet)? / Have you (ever) had pickles for breakfast?

When you specify the time of the action, it should be simple past - Did you go to that party last week? Did you play with dolls as a kid?

Simple past refers to a "closed" opportunity - you can't do that particular action anymore (that specific party was last week, it's over / you are no longer a kid, you can no longer play with dolls as a kid).

Present perfect is more open ended and vague - it refers either to something you have/haven't done in the past, but which you could still do in the present, or to an action in the past without specifying exactly when (because it is not important in the context).

Anyways, that's just what grammar books say; but I guess rules are not written in stone and native speakers often use the tenses interchangeably.