Guys, I thought I should share this process with you. I already applied it to one of my image and got decent result, could be found here.http://photographyatnewenglandmoments.com/index.php?topic=20738.0
I saw it in when browsing a photo magazine, could not remember the name. The credit should go to them.
1. Choose an appropriate photograph (very important). I shot the following specially for this tutorial.
2. Create a new layer. You may name it "rain layer"
3. Fill it with 50% grey (Edit>>Fill) for a realistic effect
4. Add noise (Filter>> Noise>> Add Noise), choose an amount whatever suits you.
5. apply levels adjustment to make the noise prominent
6. Apply motion blur (Filter>>Blur>> Motion Blur). Choose direction of rain by adjusting the angle. The force of rain could be adjusted by radius.
7. apply levels adjustment again to make rain more visible
8. (Optional) you can again apply a low radius Gaussian blur. Do not overdo it.
8. Change opacity to blend in.
Here is the pictorial procedure
The completed image
You could have some disturbances along the edges (very thin line with more brightness could appear). crop it out or use transform tool. Hope this will help.