Dictation on the iPad is great.
I can use Dragon Tree etcetera but would like to use an iPad like voice dictation program instead for simple tasks.
Can I do the same thing on my OS X 10.7.4 iMac?
iOS like Dictation...
...currently isn't a native feature of OS X. However, Mountain Lion's new features willl include a system wide speech input.
Mountain Lion will come out this month (July '12). So you might wanna just wait a few more days.
Here's the description by Apple:
MacLife also states how to use Dictation: