I tried to burn some data to a CD-R and had the same problem. The blank CD didn't appear on the desktop or in the Finder sidebar on Lion. But iTunes launched and asked if I wanted to burn a playlist, so the system was detecting the disc.
Then I remembered that I had changed my CDs & DVDs setting in System Preferences to launch iTunes when a blank CD was inserted. I opened System Preferences -> CDs & DVDs and chose "Open Finder" for the "When you insert a blank CD:" setting.
Solved the problem for me. Now when I insert a blank CD-R, it shows up on Desktop and also in the Finder sidebar. Dragging folders to it, then clicking the "radiation hazard" burn icon burns the CD-R now. (Late-2008 first-gen unibody MacBook Pro, Mac OS X 10.7.1, stock SuperDrive, Philips CD-R media.)