Off the top of my head, I'd think that the UI would be of secondary importance in any startup that is writing systems software: if you can solve an important business problem, a fancy demo is impressive (and important!), but the basic problem you're solving often doesn't require a UI. For example, enterprise data integration (conversion between data stored in heterogeneous formats and systems), or natural language translation/processing. Many startups in information management / data warehousing: if you build a Teradata killer, the UI is certainly a component of your product, but it is pretty secondary. A startup building hardware (say, networking stuff) might not need to pay much attention to the UI, depending on the target market. etc.

If by "startup" you just mean "implementing the Web 2.0 idea-of-the-week", then yeah, I agree UI is usually pretty important.