As you can see, WebM has a huge advantage when it comes to browser adoption.
IE users are notoriously slow at upgrading, and IE9 will only be available for Windows Vista and Windows 7. Half of all Windows users will not be able to upgrade to IE9!
On the other hand, recent versions of Opera and Chrome already support WebM, users are upgrading to these versions much faster than they are upgrading Safari or IE, and Firefox 4 is likely to be released in the near future, thereby boosting the market share of WebM-supporting browsers to possibly more than 30-40%.
This leaves Safari basically carrying the torch for H.264 with little help from Internet Explorer. Within the next 12-24 months, H.264 might be lucky to have a market share above 10%!
As a final nail in the coffin, IE9 and Safari, being bundled with operating systems, are likely integrated with those operating systems in a way which makes adding WebM support as simple as installing it as a system codec.