This Check checks status of an ADSL line via the SNMP ADSL-LINE-MIB. Written for and tested with a DrayTek Vigor165, should work on other DrayTek ADSL routers/modems and probably with other hardware (feedback is welcome).
If want to try this with other Hardware than Vigor165/160/130, you will need to adjust the snmp_scan_function.
The ADSL line is usually on interface #4, although this should be autodiscovered by the plugin.
New Version (>1.1) requires CMK >2.0.0! If you use an older version, look in the v0.7 branch (but be warned, this code is really ugly).