I have a WIFI laptop, no bluetooth. My cellphone has 3G data plan. Is there a way to connect my computer to my phone to get internet access? Can I do this instead of paying for separate internet service in the house?
I know people who've done this actually when their neighborhood internet lines went dead for the better half of a week. The problem is, its hard to help you without knowing what type of phone and service you have. They're not created equal.
As SuperFlyer pointed out, some carriers claim that "unlimited" is just a marketing term, and in fact they have a limit to how much data you can get away with using per month. T-Mobile and Sprint are more likely to turn a blind eye as long as you pay every month.
Meanwhile, many WiFi enabled Smartphones (such as Nokia Symbian or Windows Mobile) allow you to install applications that will broadcast your cell's data connection over WiFi. Your laptop or any other device in range will see it and connect just like a home router (I do this when I travel... the wife hates it on camping trips! I call it WiFi in the woods).
But despite all that, I much prefer to have a dedicated internet line in my house than to rely on my phone's data connection. Why? Well, first of all a home data connection is actually faster and more reliable (less latency ie. lag in response). Secondly, I want my computer to be accessible online when I'm not home so I can use remote desktop services, and to power my VOIP phone service. Thirdly, most US networks can't handle Voice and Data at the same time, which means when you're using data, you won't receive any phone calls, and vice versa. Imagine if I'm uploading pictures to send to my family across the country, and suddenly I get a phone call... the internet connection goes on hold and eventually the computer will think I've disconnected and give up.
The bottom line here is that yes, it is possible, and comes in extremely handy when travelling or if in a pinch / for emergencies. But I would still strongly recommend having a real dedicated internet line if you can.