Answer:
What you need to do is use Host Headers and Alternate Access Mapping. You could even use different port numbers because ISA Server can easily redirect external users to the right port for your SharePoint site. First, I'll show how to change the URL (host header) and then show how to add a different link for accessing the intranet site internally.

If you've configured your SharePoint site with a specific URL (say, http://webportal) and now you want to change it to another URL (say, http://sharepoint.seattlepro.com), you can use the Alternate Access Mapping feature in MOSS 2007. Here's how.

1. Start SharePoint 3.0 Central Administration.
   
   
2. Click on the "Operations" tab.
   
   
3. Under the Global Configuration section, click on "Alternate access mappings."
   
   
4. Click on "Show All" in the upper right-hand corner and then click on "Change Alternate Access Mapping Collection."
   
   
5. From the "Select an Alternate Access Mapping Collection" window, click the URL that you'd like to change (in this example, http://webportal).
   
   
6. Click "Edit Public URLs."
   
   
7. Change the URL listed in the Default box to the one you want. So, in this example, change it from http://webportal to http://sharepoint.seattlepro.com.
   
   
   
8. Click Save.
   
   
9. The next step is to update the information in Internet Information Services (IIS). Start IIS Manager. This example assumers you're running IIS 7.
   
   
10. Highlight the Web site with the URL you want to change.
    
    
11. In the Actions pane, click "Bindings" (or right-click the Web site and select "Edit Bindings").
    
    
12. Highlight the entry in the "Edit Site Binding" window and then click "Edit."
    
    
13. In the Host Name box, enter the new URL you'd like to use (for example, sharepoint.seattlepro.com). Don't type "http://" before the host name.
    
    
    
14. Click OK, then click Close.
    
    
15. Go to the command prompt and run iisreset to restart Internet services.

You should now be able to use the new host header and access the site with the new URL (http://sharepoint.seattlepro.com).

Because you're using ISA Server to publish your SharePoint site, you need to make sure that you update the DNS server and the ISA Server rule that allows you to access the Web site from the external network. For example, you need to add a host record for sharepoint.seattlepro.com in a DNS server that's accessible from the Internet and add the URL sharepoint.seattlepro.com on the Public Name tab of the ISA Server rule that's publishing the Web site.

You also want to use a different URL for the intranet site internally. Let's say you want to use the URL http//intranet. You can add this URL in Step 6. On the Alternate Access Mappings page, instead of clicking on "Edit Public URLs," click on "Add Internal URLs." In the Add Internal URL box, type http://intranet, then click Save.

You still need to go to IIS Manager and in Step 13 add a Site Binding for intranet. You don't need to type "http://" in the Host Name box; Simply type the host header that you want to use, which in your case is just the word "intranet" without the quotes.

Make sure you don't forget Step 15 after modifying the bindings.

That's it! Now your users can access the intranet site internally by typing "http://intranet" and externally by typing "http://intranet.seattlepro.com."