What kind of experience is necessary to develop content or applications for use with uPortal?

It depends upon what kind of content you want to deliver through the portal.

RSS

For simplest content provision, you only need to know how to produce an html document. One of the defined kinds of channels for use with uPortal is an RSS (RichSite Summary or Really Simple Syndication) channel, the format by which many sites, products, and services expose content. If you create an XML document which adheres to this standard, the uPortal framework will handle formatting and presenting the output for you.

XML

RSS is a sufficient format for many needs, but some problems require a more complex or at least more domain-specific XML format. uPortal supports rendering XML via XSLT. To implement this sort of channel you'll need XML and XSLT skills.

Web Proxies

Note that your expertise need not be Java expertise. uPortal ships with a CWebProxy web proxy channel capable of proxying remote web content. You can develop your portal application using PHP or .NET or your choice of technologies, adhering to some rules so that your content is suitable for proxying, and then you can configure a CWebProxy instance to present that content in the portal.

JSR-168 Portlet

If you do have Java expertise, you could create a JSR-168 portlet using Java. Your portlet might require SQL or other RDMBs skills, or it might require skills accessing Web Services, or it might require other expertise, depending on what you're looking to do.

NOTE: Add link to Andrew's forthcoming article on channel types...