I can't tell you how to fix it, but I can give you a couple of
work-arounds. I have been having this problem for a couple of weeks.

You can drop back to the Update Manager window and click the "Check" button. It
will fail with a signature validation error, but if you click the "Check" button
again, it will run without errors. Alternatively, you can open a terminal
session and enter "sudo apt-get update". When you run the command, it will fail
with a signature validation error, but if you hit the up arrow and run the
command again, it will run without errors.

My guess is that dropbox has some issues.

Yea, Ive noticed it for a few weeks too. It was a different app before
Dropbox before but now its been dropbox for a week or to.

Ive noticed that too that I can get around it but, since its every time
I was wondering if it was something else.

I found in the forums someone said to use 10.10 instead of 10.04 in
checking for updates but, since 10.04 is LTS I would rather not.

The standard thing to do is open a terminal and type: sudo apt-get -f
install

I wonder about how you installed Dropbox. It is looking for a key and isn't
finding it. That suggests that you have added Dropbox to your sources list.
Could you tell us what you did to install Dropbox? That may lead to a fix
for the problem.

In my case, I hadbeen running Dropbox for quite some time before this message
started popping up.