Amazon.com is now listing HeartGold and SoulSilver as having release dates of Sunday, March 14th. Previously, the site did not list any date, so perhaps they have just received official confirmation from Nintendo. Diamond, Pearl, and Platinum were also released on Sundays. Additionally, Amazon sent out an e-mail to those who pre-ordered the games stating that the estimated delivery date is between March 14th and 16th (the date depends on where you live).
The March 14th date also fits the theory posted yesterday that Nintendo is using the HeartGold & SoulSilver TCG set to advertise the games in February, and will then release them in March for maximum sales, as was the case with some TCG sets in the past. Of course, until an official source confirms the March 14th date, we cannot know for sure if it is tentative or not. Stay tuned!