It seems that they might be available on arrival, but I'm not sure if you have to have a letter of invitation first. There is not much information online and due to the climate there at the moment they might not be giving visas at all regardless of what it might say online.
I'm a British citizen, travelling with a Swedish citizen, but generic answers that apply to all nationalities are also OK.
From the Lonely Planet web site:
To obtain a Libyan visa, you’ll need an invitation from a Libyan tour company. The tour company will then send you a visa number. Make sure you have an Arabic-language confirmation to smooth the process with airlines, the embassy or immigration officials. You can collect your visa either from the Libyan embassy in your home country or at your entry point to Libya, but specify which you prefer when making contact with the tour company. The process generally takes two weeks, but allowing for a month is safer. Visas are valid for 30 days from the date of entry.
There is also a thread on the Thorn Tree forum about the current (late 2011) status of getting a visa. It seems the Libyan consulates are currently not issuing any visa.
