There's a nifty App out there called MailTags (http://www.indev.ca/MailTags.html). I've been using it for a year or so on Lion and it connects your mail to iCal pretty well.
From what I'm reading on the MailTags support forum, however, the Mountain Lion version of MailTags, though available, is causing some pretty bad crashing problems at the moment. I'm personally not installing it until I see those issues resolved. I would keep an eye on it and wait for MailTags to fix the bugs, then take it out for a spin - they have a free trial.
In Lion with MailTags 3, you would open an email in Mail, click the tag and either assign an iCal Event to the email or an iCal Task. You made the assignment right inside Mail using the "Tag" and the Event or Task would automatically appear in iCal - and Event/Task would include a link back to the Mail message it came from (which was super convenient).
Tasks in iCal seem to be gone since I installed Mountain Lion tonight. I'm not really happy about that - cannot figure out how to get them back. The "Reminders" icon was added to my toolbar - I'm guessing that Apple killed Tasks in iCal and replaced with Reminders?? Though I'm not convinced I didn't lose some data in this process.
It will be interesting to see how MailTags for Mountain Lion works with these changes from Apple - once MailTags for Mountain Lion gets stabilized anyway. I was a PC person for 20 years. The only thing Microsoft does better than Apple is Outlook's full integration of calendars, tasks and mail. Wish Apple would wake up and see the value to integrating these tools into a single app - bouncing between THREE apps now for this is not productive. But MailTags is the closest thing I've found to bringing that integration together since I went Mac. Hopefully they'll have the new release of Mailtags for Mountain Lion stabilized soon, cause I'm already missing it.
