Its only been a few months since the Desire was released (and actually available/in stock) on Softbank, and they are already taking orders for the Desire HD. If you jumped on the Android bandwagon and bought a Desire, its not quite worth the upgrade unless you have more money than you need.
These are the HD ‘improvements’
Bigger screen: 4.3″ (Not AMOLED) Screen compared to the Desire’s 3.7
More ram & internal memory: 1.5gb internal & 768mb ram
Better camera: 8 megapixels
Better sound / multimedia: Dolby mobile and dlna.
I assume the non hardware upgrades will also be made available via custom roms for the original Desire. If you are planning on getting one, I highly recommend pre-ordering ASAP. I reserved my Desire the day they started taking pre-orders, and one month later I was one of the very few to get the Desire on the official sales day. Unless they get a decent amount of stock this time round, its very likely that if you do not pre-order, you may be waiting for a few months to get ahold of one.
Here is the Softbank link: http://mb.softbank.jp/mb/smartphone/product/001ht/
And the HTC official Desire HD page: http://www.htc.com/www/product/desirehd/specification.html