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?
...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:
Now you can talk anywhere you can type. Dictation converts your words into text. It uses the built-in microphone on your Mac, so there’s no need to set anything up — just start speaking instead of typing. When you say “comma” or “exclamation point,” Dictation punctuates for you. The more you use Dictation, the smarter it gets. It learns voice characteristics. And it recognizes people from your contacts so it enters names accurately. Dictation supports English (U.S., UK, and Australia), French, German, and Japanese.
MacLife also states how to use Dictation:
To use Dictation, open any application that has a text input area and position your cursor in that text area. Press the Shortcut key twice (by default, it’s the Function key).