It hasn't been officially released on DVD in Region 1 yet, but should eventually come out from Disney - Disney made a deal with Studio Ghibli a couple of years ago to release a number of their movies...I'm not sure if
Whispers of the Heart was part of that agreement, but it most likely was...nausicaa.net has the details.
If you want the movie now and have a region-free DVD player, or a spare DVD player to set to Region 2, I would recommend ordering it from CD Japan:
www.cdjapan.co.jp
They have a good selection, and are a reputable Japanese media distributor.
The site can be difficult to navigate sometimes, so here's a link to the DVD:
Whisper of the Heart
BTW, its Japanese title is
Mimi wo Sumaseba
Now, if you don't care about an official release, you can probably pick up all of Studio Ghibli's releases on bootleg DVDs on ebay for like $10