If you wish Apple or a third party developer to restore the sidebar to iCal in Lion, please see the first post in this conversation:
If you wish to use iCal in Lion in a more advanced way — within the constraints imposed by Mac OS X 10.7 (Build 11A511) — please read on.
First: quit iCal, then enable the Debug menu.
To enable the menu, you might use the Secrets preference pane. Or if you're happy using Terminal, run the following command:
defaults write com.apple.iCal IncludeDebugMenu YES
Launch iCal. Note the Debug menu. Proceed with caution — I don't know whether all menu options are tested by, or intended for, everyday users of iCal.
Double-clicking anywhere within a mini-month (year view, minimal window, top right) does not open the required date in the separate larger window, so this is not a replacement for the mini month and other sidebar features that have been removed. But it's food for thought.
Not dependent on the Debug menu, you may prefer to use keyboard shortcuts to work around the absence of the mini month.