It may cost you a fortune and almost certainly more than getting a broadband deal. Have a close look at internet data charges. See how much a 3G dongle costs. I would not connect more than one device to a phone.
I don't mind them making a profit but deliberate restriction on the use of technology through extortionally high charges is immoral. Except for those fortunate enough not to have to live on a small budget.
thanks for your posts
Buy an Android phone with v2.2 (Froyo) or later (2.3 is Gingerbread). This will allow you to operate the phone as a WiFi hotspot and you (or others) can log onto the internet using the WiFi facility on your computer device. Android Tethering now supports Infrastructure so you can have the hotspot running 802.11g with WPA encryption. You can also use MAC filtering to exclude unwanted users. I use my ZTE Racer phone, running v2.3 of Android, as a WiFi hotspot to allow my Android tablet to access the internet when I can't get free WiFi. I can also connect my W7 laptop to the Android phone with WiFi. I've noticed many iPhone users trying to piggyback onto my Android Tether without success.
My ZTE Racer is on '3' and was free with a contract costing £12/month. This gives phone/SMS/internet (the latter is 500 MB/month and is plenty for occasional use).