Hi,
I'm sure you're long past this migration by now, but I figured I'd reply anyways. This link:
http://technet.microsoft.com/en-us/library/gg576862(v=exchg.141).aspx will help you remove the last 2003 server from your 2010 environment, but to answer your question,
no, your users will not lose access to public folders. Run this script from 2010 EMS C:\Program Files\Microsoft\Exchange Server\v14\Scripts\MoveAllReplicas.ps1 -Server <2003 Server> -NewServer <2010 Server>.
That will run for a bit, but to verify that all replicas were removed from 2003, log on to your 2003 box, open up ESM, expand out Administrative Group, <Your Admin Group>, Servers, <2003 Server>, <Storage Group>, Public Folder Store and
select Public Folder Instances. When that no longer shows any entries, you're safe to move on to the next step of removing 2003. Like I mentioned, the article I pasted is a great article to ensure you don't miss any steps, like moving your
Public Folder hierarchy.
I'm sure you're done, but maybe this will help someone else.