Well... the Lumix Vario 14-45mm lens has a filter diameter of 52mm so I took the plunge and bought center-pinch lens cap with the leash on ebay for a few bucks. Hope it works!
edit: FWIW, this is what I bought, and it worked just fine with the 14-45mm DMC-GF1 lens:
"52 mm front lens cap for Nikon Canon Sony w/ leash"
at this ebay store:
http://stores.ebay.com/qualshot
It cost like $7 including shipping, a little more than some others, but the leash was thin enough that I was confident it would go through the strap loop along with the included camera strap, and it did.
It's a bit tricky to put on. You have to attach the leash to the camera body first (using the end with the clamp), then take the lens off the camera body, then attach the other end of the leash to the lens cap by feeding the whole camera body through the loop. sorta a brain teaser.