How can I upload HD video from iPhone 4 directly to web?
Pixelpipe
From https://webapps.stackexchange.com/questions/3486/is-there-a-way-to-upload-higher-resolution-videos-to-youtube-or-any-video-sharin

This free app allows iPhone 4 users to
directly upload their 720P videos to
YouTube, as well as Flickr, Facebook,
MobileMe, Viddler, Posterous and a
whole slew of other video sharing
sites. The app evens works via email
and FTP. The only caveat is that the
videos cannot be over 200 MB in size.

Regarding with how to use it, check out this post:
http://islandinthenet.com/2010/07/06/pixelpipe-upload-720p-hd-video-from-iphone-4-to-youtube/
I recommend that post, it gives some great information on the app and how it is used.
For example:

A “pipe” is the
connector between the PixelPipe app
and one or more destination sites
where you want your content to be
uploaded. For example, I set up pipes
for flickr, picasa, and Dropbox and
many others.
I setup some of my pipes as default
routes. This means that any media I
select to upload will automatically go
to these default pipes.
For each pipe
I created, PixelPipe created a routing
tag. The routing tag explicitly tells
PixelPipe where to upload the content
overriding any default routes. If you
want your content routed to different
photo sets you can create a routing
pipe and tag for each one.

Also, given this:

since other developers are sure to create a similar product now that they know it is possible to upload 720p videos.

If you don't like the PixelPipe app, it shouldn't be too long before another solution pops up :)
According to this page, an app called SmugMug is also 720p uploading enabled, however I couldn't find much in reference to that on their site.
http://www.tuaw.com/2010/07/01/want-to-upload-720p-from-iphone-4-try-pixelpipe/