Direct flash embeds are not allowed at wordpress.com for security reasons although some flash can be used with the gigya shortcode. One thing to remember about the gigya shortcode though is that it is not an officially supported thing, which means that it could go away at any time for any reason.
If you want full flash compatibility, you would have to self-host a wordpress installation on a third-party web host such as bluehost, dreamhost, etc.
Given that Apple iphones, ipads and ipod touches and the latest version of Android will not get flash, I would be seriously reluctant to use much flash on any website. Adobe has killed mobile flash and I give flash probably 18 months before it is dead.