Adam Nash, a very smart friend of mine from college who writes the great blog Psychohistory, recently mentioned that he thinks it's a great sign when a developer goes cross-platform almost immediately, citing the Joost beta for Mac OS X.
I started writing a comment in reply, but it started getting long enough to warrant a post and trackback.
As usual, Adam's reasoning is sound, especially considering his background in VC. He's right that a company that can build great cross platform apps simultaneously probably has a great development team. However, I'd argue that cross platform development rarely produces the best products.
First, the product often winds up representing the lowest common denominator of the capabilities of the OS' they serve. They are often not as polished or well-integrated as native apps. Firefox is a good example of this from a UI perspective. While it's certainly a pretty well-written app, it's not as native-looking on the Mac as Safari or on Windows Vista as IE7. In both cases, Firefox is a bit out of place. (Read this post on Coding Horror for a similar opinion.)
Also, in order to ease development, cross platform apps often have intermediate layers to factor out the underlying OS. These layers can impede performance and may prevent the app from taking advantage of native services like DirectX or Quartz. The resulting apps aren't usually as fast as their native counterparts. Microsoft's Mac apps certainly ran into this problem when writing cross platform "core code" apps on our Windows Layers for Macintosh (WLM) back in the mid '90s (anyone remember Mac Word 6?)
Finally, developing cross platform reduces the overall innovation a developer can provide. Building for multiple operating systems (or browsers) is never less work than building for one. The time spent architecting, coding, testing, and debugging for multiple platforms is time not spent adding new features, making the product more reliable or secure, or satisfying other user demands (or saving investors' money).
There are certainly no guarantees of a gorgeous, OS-exploitive, fast application when you target only one OS, but its's way harder when you are trying to serve multiple masters.
There's no doubt that teams that can execute cross platform consistently well over time are probably great, but just think what they could accomplish if they chose to focus all that talent and energy on one platform.
Anyway, go read Adam's blog. Lots of good stuff there, especially his financial posts.