I was looking for this kind of app back when I had my Palm Treo and never came up with it. Don't think there is a big enough market for it (my job is 100% linux software development and I can't believe I am saying this).
Posted from my CrackBerry at wapforums.crackberry.com
I'm not sure what SharkModem does specifically different with/without the tether plans, but I was just able to use berry4all on an Ubuntu netbook without a problem on T-Mobile.