For the osmosis docs, [I see the command option](http://wiki.openstreetmap.org/wiki/Osmosis#Extracting_bounding_boxes):

    --bounding-box top=49.5138 left=10.9351 bottom=49.3866 right=11.201

for PostGIS you can use [ST_MakeEnvelope(left, bottom, right, top, srid)](http://postgis.refractions.net/docs/ST_MakeEnvelope.html) to build a bounding box, then the [`&&` bounding box operator](http://postgis.refractions.net/docs/geometry_overlaps.html) to find where the bounding boxes intersect:

    SELECT *
    FROM mytable
    WHERE mytable.geom && ST_MakeEnvelope(10.9351, 49.3866, 11.201, 49.5138, 4326);

The SRID 4326 is for WGS84 Lat/Long, and is only required for PostGIS 1.5; it can be omitted for later versions.