USB connections.Currently the cheapest option of ADSL is available as a USB interface. This uses the point-to-point protocol over ATM (PPPoA). Unfortunatly PPPoA is not available on Macintosh and is only just available in Linux. The drivers for the USB ADSL modem (from Alcatel) are proprietary. Therefore it seems even more unlikely that anyone using an OS other than Windows98 and Windows2000 will be able to use the cheapest £40 offering of ADSL available at this time.
Please see http://www.nts.com/library/why_pppoe.html for more information on Architectural Options for Subscriber Access to Broadband Services.
The FAQ for Operating Systems from the USB modem producers, Alcatel, can be seen here.