15M+* iPads have been sold. If your website is built using Flash, that's 15M+ potential customers you're alienating. Read why Apple's mobile devices don't support the tool: Thoughts on Flash, by Steve Jobs.

The Flash logo

Additionally:
Apple will sell 50 million iPads a Year, on Business Insider
Top 10 Luxury Brands' Sites Fail To Work On iPad, on PSFK
Flash and SEO — Compelling Reasons Why Flash & SEO Still Don't Mix, on SEOmoz
Things to Know About Flash Websites, on Don't drink the koolaid
Don't Build Your Site With Flash, on Hobo-Web
Flash, iPad, Standards, on Zeldman.com
An Adobe Flash developer on why the iPad can't use Flash, on RoughlyDrafted

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Update:
Flash set for mobile exit, on .net magazine
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*15M figure obtained by doing the maths from here, here, and here.

Flash logo via Shirtoid.