ON A WALL outside the Harvard Housing Office in Cambridge, Massachusetts, an anonymous street sage has inscribed the following day-glo painted message: "HELL is looking for an apartment in Cambridge. HEAVEN is finding one under rent control." Anyone who has had to look for rental housing in a rent-controlled community will detect more than a kernel of truth in this graffiti: Such a search can involve weeks, and sometimes months, of combing want ads, pounding the pavement, and finding a busy signal on the lines of landlords already besieged with callers seeking their coveted units. But for the chosen few whose search ends successfully, the payoff is heavenly: A lease on a rent-controlled apartment entitles the lucky bearer to accommodations that are 30 to 60 percent cheaper per month than an identical unit in the uncontrolled part of the market.