A regular 10Base-T or 100Base-TX connection only requires those two pairs, and yes, they are carried on pins 1/2 and 3/6 of the modular jack, usually connected using the white/orange and white/green pairs (see the recent 568A vs. 568B thread in this area for the debate over which way around).
The newer 1000Base-T networks require all four pairs, however, as do some 100Base-T variants which were intended to run over CAT3 cable. You're probably not likely to run into the latter though.
