I have a Vodafone contract with unlimited broadband for £5 a month added to my mobile bill- hook up the mobile to the laptop to act as a modem via usb or bluetooth.
3G service gives a few hundred K (think early broadband speed), good enough for web browsing and email fairly quickly. If your phone has it and you're in an area that provides it, HSDPA is quicker (think entry level home broadband - when you have a good signal - but should be improving), it's fairly new so not all phones have it and it doesn't cost any more when it's on. When in rural areas it should default back to GPRS, which is equivalent to the old 56K dial up modems.
If that's all a bit complicated, most providers do the dedicated usb dongle or PCMCIA card, but have their own contract seperate to your phone, I think about £15 a month - but simple to use.