How "sophisticated" are your users?
The reason I ask is: it's fairly simple to password protect the VBA code.
Once you've done so, you can create a

Private Sub Worksheet_Activate()

sub that basically selects another sheet...
The only problem with this approach, is that if the users are just a "little"
savvy,
they can simply create a VBA sub in their own personal.xls file
that does:

Application.EnableEvents = false

and the sheet is "unlocked".

Another way is to do as you said, and make a lot more call to the
protect/unprotect functions  for the sheet.

I was just about to post that I found the simplest way to
protect/unprotect the sheet for data  entry into the database, but
the code  contains the password and the code is viewable and
editable. Is there a way to hide this? I was looking at encryption,
but I don't really understand how that works.

The users, just fyi, are engineers from varrious disciplines,
including cs. I would like to protect this using zimmerman's pgp,
but that would probably defeat the purpose.

Ay suggestions?

No additional code  needed.

From the VB Editor, set the Visible Properties on the data  worksheet
to Very Hidden. Your Form can still write to a hidden worksheet. In
the VBE, look at the VBAProject Properties for your project
(Right-Click on your project) and Lock the project  for viewing.

Anyone savvy enough - and mean enough - can still overwrite your data
but it'll make it much more difficult to do so. Most Users that I
work with don't even know that there is a VB Editor.