If i want to add a Neutral Density filter to my lenses for GH2 what is the thread size? Is it 52mm on all Panasonic Lumix Lenses or does it vary?

It varies;

46mm
14mm/f2.5 and 20mm/f1.7

52mm
14-42mm/f3.5-5.6, 14-45mm/f3.5-5.6 and 45-200mm/f4.0-5.6

62mm
14-140mm/f4.0-5.8

67mm
100-300mm/f4.0-5.6

An option is to buy filters the size of your max. lens and then get step-up rings for your other lenses so you can use the same filters on them, too.

BTW, Polaroid is making a variable ND filter at a very reasonable price. Seen it on Amazon for $30-40. I don't have one so I can't tell you if there's a color cast or not.

I just bought the Cokin system. The P-size works for my Panasonic GH2 lenses and my Pentax lenses. You need to buy step-up adapters and all, but you just have to buy the filters once. Cokin also has hoods you can add and a matte box, but they are notoriously hard to find and reviews have been mixed.

Note, someone on the internets has noted that the Panny 20 F1.7 retracts ever so slightly when you turn the camera off, so if you have a step-up ring or other adapter flush mounted it may interfere with this retraction. Here's the link:

Micro 4/3rds Photography: Home made stand off ring (http://m43photo.blogspot.com/2010/06/home-made-stand-off-ring.html)

Thank you all, very helpful.