If you "moved" the content  using an Custom List Template (STP) it won't
show up because the underlying Site Definition for every area  is
different. If the Area Template is the same but the language is
different, it also won't show up.

We are seeing the same outcome of whether the content  was created  using a standard list, a template, or a custom list. Language is always the same.

Step 1. Created an area  A within SPS 2003.
Step 2. Created Custom list  A within Area A.
Step 3. Built a Web Part Page in Area A with web parts based  on Custom Lists A.
Step 4. Created Area B.
Step 5. Go to Modify Columns and settings  of Custom List A. Go to Select a Portal Area for a subset of lists. Click Change Location. Checkmark Area B.

RESULT: Can't create a Web Part Page in Area B using Custom List A. It will appear in Portal Listings but won't appear in Add Web Part ---> Browse of Web Part Page of Area B. Also, if we return to Custom List A and Modify Columns and Settings and Select a Portal Area, the location properties  of Custom List have reverted back to original settings.

In other words, any content created in one area is not being exposed to the list in another area. Suggestions.

This is happening probably because you are trying to move the list
template from an area based  on one site definition, to an area  based on
a different site definition. You can use a workaround to move your
lists despite this limitation.

weblogs.asp.net/.../236593.aspx

As for the "Select a Portal Area for this List" issue, what do you mean
by retain the changed properties? Does it add  the listing? Are you
maybe confusing this with a way to move content  around on the site?

Can you use the DVWP to expose this list  in area  B?

This is by design. Each SPS Area is technically a WSS Web Site (SPWeb).
Via the browser-based UI (like Add Web Parts - Browse), data in one
SPWeb is not available in other SPWebs. There are at least four ways
that List A from Area A can be viewed out of scope in Area B:

Clunky Non-List Web Part Options
1. Page Viewer Web Part
2. Web Capture Web Part (from the STSTPKPL Office Web Components on
the SharePoint CD or Microsoft download site)

FrontPage DVWP Options
3. Manage Catalog - Add
Sweet, but only for data in sites in the same Virtual Server
(should work fine for your situation)
4. Use Lists.asmx Web Service - GetListItems
IMHO, this is the best option available today

Ask follow-up questions so I can better direct your next steps,