Upgrading an Ubuntu installation has never been a safe move, much less days after a release. This time around such statement is particularly true due to all the stuff that is being introduced as part of the switch to Unity.
My recommendation? Stay away from Natty Narwhal for some months. If you want to use it, don't upgrade, use a clean installation instead, BUT...
DON'T EVEN TRY IF YOU ARE USING AN UBUNTU DERIVATIVE!!!!
The Elementary OS team have stepped forward and publicly warned their users not to upgrade to Ubuntu 11.04:
"It has come to our attention that on Thursday, April 28, 2011, users of elementary OS Jupiter may be prompted to upgrade to Ubuntu 11.04. We strongly urge you not to do so if you are prompted to; not because we do not want you using Ubuntu, but because the upgrade process may render your install unusable."
You can read the full public service announcement on the project's own WEBSITE.
Unfortunately, this potential risk applies to all Ubuntu derivatives. ZorinOS, MoonOS, Pinguy and Linux Mint users (to name just a few popular examples) could wreck their systems with this upgrade process as well!
NOTE: I am currently testing Natty, working on a review which I will publish in the next few days. From what I am seeing, Ubuntu 11.04 could use some time to mature further, so no need to rush an upgrade!