We manage quite a few OSCommerce sites for our clients and have implemented not only Friendly URLs but Ultimate URLs (much better mod). While there's typically a dip in traffic it usually doesn't take long if your site has decent
PR already (I.e. Google stops by every few days) for it to not only recover but improve. The variables must go!
AVC is correct with regards to 301s. Make sure you handle that. Note: A really nice thing about Ultimate URL package is that anytime you rename a product,
A: it changes the URL to reflect the product name (key phrase) in the URL with the .html extension
B: it handles the 301 via php for you automatically.
Best of luck with it!