if your life would be better if you had authorize.net payment gateway and US sales tax handling for the eCommerce module SilverCart, you should participate in funding these features. You thought the world would end in 2012 anyway, didn't you?
There is already one company interested in funding and we - as the module developers would also participate.
Total costs amount to ~ 2500 USD to implement authorize.net payment gateway and US sales tax handling.
In case you have no idea what SilverCart is: SilverCart is the most comprehensive eCommerce module for SilverStripe. It has proven to handle stores with > 400.000 products and is growing steadily.
You can get your hosted demo if you want to play around a little bit before jumping in.