Interfacing CMOS and TTL serial lines
A voltage divider works fine for converting 5 V signals to 3.3 V signals.  I've used 2 k&Omega; in series followed by 3.9 k&Omega; to ground for exactly this purpose.  In this case the speed was 115.2 kBaud, and the delay introduced by the resistors was small compared to a bit time.

The real problem is the other direction since the signal probably has to be amplified.  Unless the 5 V chip has "TTL" input levels, 3.3 V may not be above its guaranteed input high level.  Some parts use 20% and 80% of Vdd for the guaranteed levels.  If yours does that, the signal must exceed 4 V to be reliably picked up as high.  Level converters are small and cheap and exist for just this purpose.  Another trick is to use a HCT gate (really any gate with TTL input levels) powered from 5 V to do the conversion.  That only makes sense if you have a left over gate, otherwise a deliberate level converter makes more sense.