Creating diagonal lines in Adobe Illustrator is somewhat of a pain. I'm creating the graphics for our new Web 2.0 site, and had a hard time creating diagonal lines. But here I'll show you how to create angled 45deg Web 2.0 lines in Illustrator with relative ease.
First, turn snap to grid on.
Select the line tool, set the stroke width to 5pt (or your desired width).
Make sure your stroke color is the color you want it. In this example, my stroke color is black. Next, on the canvas, draw a diagonal line that is slightly larger than the major grid box.
Next, Move-Copy the line 12px away from the first line.
Hit the copy button. Once your copy appears on the canvas, press CTRL-D to repeat. You need to repeat until you have a major grid completely covered. Like so:
Next, select all the diagonals then go to Object>Path>Outline Stroke as shown below:
Next, draw a square (with no fill and no stroke) as the same size of a major grid box (72px by 72px) and place it over the covered major grid like this:
Your square should be 72px width and height. Next, select all, and click Divide on your Path Finder as shown below:
Ungroup and cut the excess diagonals from the major square:
Now you should have a nice cut pattern like the one above. Select your pattern and drag it to your swatches. Once you see your pattern in your swatches palette you can then use the swatch and apply a fill to your irregular pattern and archive the following results:
Pretty cool! :) Hope that helps! 
Helpful Tip:
If you want to apply a radial gradient or a fill color to your stripes, you'll notice that this method wont work. But to make it work, you just need to continue a little more. Select the original pattern you created:
Next, Merge:
(This effectively merges the original invisible (no stroke no fill) square with your diagonals.)
If you choose to stop here, you'll notice the pattern diagonals still act like individual objects when a fill is applied, and you get something like a diamond shape when applying a radial gradient:
To unify the stripes as a single object do the following (and ultimately, have the gradient applied to the diagonals as a whole, instead of individually), Ungroup:
Then Add Shape Area in your path finder tools:
Then click Expand:
Now you have the gradient applied to the whole shape instead of each individual diagonal line:
There we go! Hope that helps!