Consider this the official thread for DS Matchups for Nintendo Wi-Fi.
Post your friend codes for various games and use this thread for matching times to show up on Ni-Fi. I'm currently playing Mario Kart, haven't got Animal Crossing yet, my code for MK is:
0000-6325-1174
This is the code for a rom dump though since I haven't bought it yet. I'm going to buy it in a day or two, alongwith Animal Crossing, then I'll put up my own code =D.
By the way, Mario Kart is definitely a MUST BUY title for the DS. The online gaming experience is excellent. I'm currently using my Wireless router to get on with Nintendo Wi-Fi. Works fine all the way from India, although sometimes it does take 2-3 attempts to log in to WFC.