You can start a
TN job pretty much anytime you like. If your new employer wants you to start May 1, 2009, just date your letter on or around the expected date of entry into the U.S. You could optionally indicate an end date as well.
When they stamp your I-94 (a piece of cardboard which is your work authorization) it will indicate the actual date you entered the U.S. CBP should also stamp it 'Multiple Entry' indicating you may cross back and forth between the U.S. and Canada as much as you like on that I-94. You could date the letter the same day you are attempting to enter the U.S.
However, I know there are TNer's coming back to the Canadian border to renew their existing status 2 months ahead of time, which is just a crazy risk in my opinion. It should be no sooner than 10 days to 2 weeks before the expiry date. Some CBP Officers won't approve a new TN so early. Thus there are many others taking risks and getting away with it. Some might say indicate an end date. It all depends on how much risk you are comfortable with. Hey, some people lose their shirt in the stock market and are comfortable taking that risk. I personally don't encourage those kinds of gambles in the realms of U.S. Immigration.
You pretty well have to keep in the back of your mind that the laws under NAFTA are very inconsistently applied and have been ever since NAFTA was enacted in 1993/94. A Free Trade Officer at one port of entry might make a big issue about a start/end date while a Free Trade Officer at another port might think the duties presented are way more important and might not bat an eyelid about any start/end date.
Remember the TN is an actual 'status' not a visa. So I would rephrase the sentence as follows since the DHS can be pretty picky about the terminology sometimes depending on where you enter:
“For these reasons, we request that you issue Mr. ABC TN Status for a period of three years."