Hmmm.... I don't think that you're going to find that the way that your iPhone and MacBook connect via wifi is going to be slowing you down versus the bandwidth that AT&T makes available for a 3G phone on the cellular network. More likely than not you're connecting using what's called the 802.11n protocol. According to the specs, 802.11b is up to 11Mbit/sec, 802.11g is up to 54Mbit/sec, and 802.11n can go up to a screamin' 150Mbit/sec. Plenty fast enough for your use versus the performance of a typical 3G connection itself.

Still, the Apple iPhone iOS 4.3.x hotspot capability has the ability to let you connect using any one of wifi, bluetooth or tethered via USB, and since I've shown how to connect via wifi in the past [see iPhone personal hotspot tethering via wifi] and since bluetooth is pretty darn similar, let's look at how to get the wired USB connection to work as you request...

To start out, you need to enable your hotspot capability on the phone by going to Settings --> Personal Hotspot and turning it on.

Now, to tether (that, btw, is the fancy word for "connect via wire") your iPhone to your Mac!

Easily done: plug that baby in with the USB cord that came with the iPhone itself and, while you're at it, let it sync with iTunes so you back up everything new on the device. Done? Good. Now you'll want to open up System Preferences --> Network, which will look like this:

Notice the last of the entries, "iPhone USB / inactive". Before we explore that, however, also note that there's a convenient button on the lower right labeled "Set up Bluetooth Tethering...". That answers that question, right? :-)

To enable iPhone USB, however, it's a bit more tricky because there's no equivalent button. Instead, you need to select that option (click once) then click on the tiny gear icon below the list of connectivity options. A menu pops up:

Choose "Make service active" and then you'll need to wait a minute as the protocol handshake transpires. Initially, you'll see the worrying message:

But after a moment or two it should all be functioning happily and the iPhone USB option moves to the top of the stack, gets a little green icon (meaning "it's on and working properly") and says "Connected":

That's all there is to it. Personally, I just use the wifi connection, but if you'd rather use the wired connection, no worries. Probably best to turn off wifi while you're using it too, so you can conserve a bit of battery power on the laptop.