Take advantage of Lightroom 2’s (or Camera Raw’s) adjustment brush to create images that are part black and white and part color.
I will start with the color image below, and convert all of it to black and white except the poster and the can.
In the Develop module, click on the adjustment brush tool (shortcut K).
Slide the Saturation slider all the way to -100.
Make sure all the other settings sliders, such as Exposure, are at zero.
Set your brush density and flow are set to 100, so that you fully desaturate when you paint. Adjust your brush size with the Size slider or the left and right bracket keys [ and ]. Now paint over all that you want to be black and white (all but the can and poster for me.) Adjust your brush size as needed. To paint with more precision, zoom in and out with Ctl/Cmd + and Ctl/Cmd – or with the Navigation Panel. If you painted over an area you didn’t mean to, click on Erase or hold down the alt/option key to get the eraser brush, and paint to erase.
When you are finished, put the adjustment brush away by clicking on it again or typing K.
PS: Yes, I wish I had turned the can around before photographing this!