I thought about doing an article about this a while back but a few weeks after Adobe MAX, there is already a codification article on when to use HTML5 and when you should still use Flash.  The article also recommends when you should write native platform code.  Here's a summary for you to reflect on:

HTML5 beats Flash at:

• Typical Business Applications
• Accessability
• SEO
• General websites
• Companion apps (accompanying popular desktop apps, movies, TV shows and games)
• Branded Microsites

Flash still beats HTML5 at:

• High-End entertainment
• Advanced 3D gaming
• Long-form video playback
• Content requiring DRM
• Faster time-to-market and ROI
• Leveraging OOP

 
When you should write native code:

• For high performance
• Getting a consistent look-and-feel with the platform

WDYK?

Personally, I disagree that you have to go native to get a consistent look and feel.  Appcelerator's Titanium and PhoneGap can actually make apps build with native web languages look pretty consistent on various mobile platforms.

Source - Darryl Taft, eWeek