Update: Opera Mobile 10 for S60 apparently does not support plugins yet. As it is a beta, this might change in the final version.
If you are wondering whether Opera Mobile 10 for S60 supports Flash or not (and if not, if it will ever support it), the short answer is (as far as I know):
You need the Flash plugin.
No browser supports Flash natively, since Flash is a plugin. This means that as long as a browser supports plugins and a Flash plugin is available for your platform, it should be possible to use the plugin to view Flash content. Some browsers also use a remote server to handle the plugin content, and then passes it on to the browser client in a format suitable for that browser. (Opera Turbo does not transcore plugin content (such as Flash), so that will not get you Flash support either.)
As I understand it, Opera Mobile 10 does (or should) support plugins, but I'm not sure if a Flash plugin is available for S60. If it is, and it doesn't work in Opera, it's probably a bug in either the plugin, or in our plugin support (report bugs in Opera Mobile for S60 in the Symbian forum).
If anyone knows about a Flash plugin for S60, let me know.
So should Opera bundle Flash, if possible? Probably not. We did that on Desktop, and that led to all sorts of problems, such as security flaws in Flash forcing us to release updates to Opera. The Flash plugin is also a pretty large download, and that is a major reason to keep it as a separate download.
And of course, Flash is a closed, proprietary, inaccessible technology which actually puts everyone at risk because it's a single point of failure and therefore a very tempting target for virus and malware authors.