I'm not sure if this is well-known or has been answered elsewhere already, but multiple Google searches have not shown this solution on how to create a line of best fit in Numbers '09 (there is another method that involves drawing a line yourself, but this is much easier and more accurate).
I stumbled across this trying the above self-draw technique and got so excited I wanted to post the technique. So here is what you do:
1) Type out your X and Y data
2) Select the data and select Charts, then select the Scatter Plot
3) Open up Inspector and select the Chart tab
4) Select the 'Series' tab below the picture of the Scatter Plot and the 'Chart Colors..' button
5) At the bottom of the Inspector window, open the 'Advanced' arrow
6) Select the 'Trendline' tab and select the 'Linear' trendline from the drop-down bar
That's it! You can even select the options to show the line equation or the R-squared value!
Here is a picture of the selected options and what your graph should look like (check that nice R-squared value!).
http://i.imgur.com/AvfPv.png
I hope this helps and hope it moves up to the top of the Google search page for this feature.

How do I access the curve fit coefficients? For example, I want to curve fit a 2nd order polynomial to a set of data and display the coefficients in separate cells then evaluate the curve fit at a defined X value (say cell B5); i.e. y = A5*B5^2 + A6*B5 + A7. So how do I extract the coefficients without copy and paste and get them in column A?
Next time around I drag and drop in new data and my new Y value is immediately calculated. And I want to do this without a macro. Hopefully I was able to sufficiently describe this.