Two days ago, Amazon.com updated their listings for the games HeartGold & SoulSilver to include a March 14th 2010 date. At that time, this was shrugged off as retaillers often put their own date guesses up. However, last night, and thanks to all who let me know, they updated with the US Boxarts which seemingly validate the date. It is possible that the date will change between now and then and it isn't confirmed. The Box Art however confirms that the PokéWalker peripheral will be included with the games.
Alright, I got this information from Serebii.net. I am shocked to see that its release is believed to be in March, just as Platinum was.
Subject: HeartGold/SoulSilver Date 
Thu Dec 31, 2009 10:44 pm