I know there was an article awhile back stating all the 737s will go, looks like tomorrow is the last one (sorry if this is a repeat, but I didn't see any other articles recently):
United's N331UA to operate the last 737 flight
United Airlines (Chicago) will operate the last Boeing 737 revenue between Salt Lake City and San Francisco tomorrow (October 27, 2009) with 737-322 N329UA (msn 24149). The pictured 737-322 N331UA (msn 24192) has been selected to operate the last UA 737 flight after more than 41 years. N331UA will operate a "farewell flight" on Wednesday October 28, 2009 between Washington (Dulles) and San Francisco via planned extended stops at the Chicago (O'Hare), Denver and Los Angeles hubs. N331UA will depart IAD around 0600 and arrive at SFO around 1800 on the historic goodbye flight. Both airliners will ferry to Victorville, CA for storage the following day.