Roomba's IR sensors are used for cliff detection, wall following, and, in 500 series and higher, for (somewhat) longer-range obstacle detection.
The reason that the emitter and detector can't "see" eachother is because these sensors work by bouncing the IR light off the object to detect. Returning light at the detector indicates the presence of an object reflecting the light.
The emitter and detector are in seperate "tubes" mounted at 90 degrees from eachother, with the meeting point of the two "tubes" being the only place where an object would reflect the light into the detector.
This part, for the cliff sensors, is underneath the front bumper. By turning Roomba upside-down, one can observe the four rectangular openings under the front bumper, and by looking in at an angle, you may be able to see the emitter or detector.
To answer the last part of your question, the foreward-facing IR sensors are not used for "bump" detection as a means to allow Roomba to push through curtains and couch skirts and clean underneath, since they aren't hard objects and can be pushed out of the way. If an object is not moveable, it triggers the bump sensor.
I hope this answers your question :-)