How can I snap lines in PostGIS?

You can use ST_RemovePoint(line, index) and ST_AddPoint(line, point, index). First, get first and last point with ST_pointN(line, index) and ST_NumPoints(line), then use ST_Snap to those points. Then use returned points to replace the old ones.

These functions should be in Postgis < 2.0 ST_ReplacePoint hould be in Postgis 2.0