Pichu was formerly available at game stop but Jirachi has taken its place:
Games Obtainable With: Diamond, Pearl, Platinum
Available from: February 27th to March 13th
Shareable?: No

This event gives you a special Jirachi when you connect to Mystery Gift at stores. This Jirachi also has a special function when transferred to the upcoming Heart Gold & Soul Silver games and their use in PokéWalker. The details of it are as follows:

Jirachi
Level 5 OT: GAMESTP Trainer Memo:
??? Nature.
Date of Obtaining
WiFi Event. Apparently had a fateful encounter at Lv. 5 Attacks:
Wish
Confusion
Rest
Draco Meteor
Ribbons:

Classic Ribbon
ID No: 02270
Hold Item:
Liechi Berry

Pichu is now available via wifi in North America and Europe smile

Games Obtainable With: Diamond, Pearl, Platinum
Available from: March 5th 2010 to March 25th 2010
Shareable?: No

This event gives you a special Shiny Pichu to represent the Pikachu Coloured Pichu of the movie. This Pichu also has a special function when transferred to the upcoming Heart Gold & Soul Silver games. Click here for further details. The details of it are as follows:

Pichu
Level 30 OT: SPR2010 Trainer Memo:
Jolly Nature.
Date of Obtaining
Wi-Fi Event. Apparently had a fateful encounter at Lv. 30 Attacks:
Charge
Volt Tackle
Endure
Endeavor
Ribbons:

Classic Ribbon
ID No: 03050
Hold Item:
Everstone

So get your wifi on and get that pichu wink or travel to gamestop and get your jirachi... or both biggrin