According to ubuntu 10.04 schedule, it was released yesterday, but Steve Langasek  announced that it will be delayed one more day to Friday March 19 ,

https://lists.ubuntu.com/archives/ubuntu-devel-announce/2010-March/000695.html

Dear developers,

Owing to some late image rebuilds for a couple of critical issues that we
didn't want to leave our beta users subjected to, validation of 10.04 Beta 1
is still in progress.  This means that Beta 1 will not be released on
Thursday, March 18 as planned; it will be released tomorrow, March 19.

To help us release the beta with confidence tomorrow, please consider
participating in the ISO testing today, as described at
<https://wiki.ubuntu.com/Testing/ISO>.

Also, as you're going through ISO tests, please also help us fill in the
blanks on <https://wiki.ubuntu.com/Testing/Plymouth> so we can complete our
understanding of the remaining boot-time issues affecting Beta 1 and make
Beta 2 truly spectacular!

Thanks,
--
Steve Langasek
On behalf of the Ubuntu release team

Fixed a lot of bugs since alpha 3 version, improved nvidia graphics driver, ubuntu one syncing, new themes, we reviewed in ubuntu fresh look.

Will review it as soon I done download it.

also you can Download ubuntu 10.04 beta 1 from here

And more information, mirrors  Here