The 180th meridian is the line of longitude 180, which runs mostly through the Pacific Ocean from pole to pole.
In theory, it should be a meridian like any other. In OSM, there's two copies at ±180° longitude, and they mark the end of the world.
Here's the mapnik rendering of the 180th meridian passing through Chukotka.
There's a lake in Chukotka that crosses the meridian at http://openstreetmap.org/browse/way/31254026.
This section should summarize to what extent various tools deal with the 180th meridian gracefully.
All layers on the slippy map appear to have problems with rendering coastline near 180°, and with ways crossing the 180th meridian. The data browser doesn't handle ways crossing the 180th meridian nicely due to a limitation in OpenLayers.
The API map call does not allow for bounding boxes crossing the 180th meridian.
All editors treat the 180th meridian as the end of the world, hence have problems editing data that crosses it.