At least one member, Allen Lau, has successfuly mounted 18" wheels on his 1G, using Racing Hart Tracer 18 x 8.5 wheels with a 41mm offset. Allen described the wheel offset as being a key point in fitting the wheels; any more than a 41mm offset and the tires will hit the strut towers. Less than 39mm offset may result in suspension problems, as the stock offset is 46mm. Tires used were 235/40/R18. 18 x 8 wheels with 225/40/R18 tires were reported to be equally good.
Several members have mounted 18" wheels on 2Gs; it is easier, since 2Gs were designed with factory 17" wheels. Jeff Mitchell reported that 18x8 wheels and 245 tires fit, but stuck out a little. See the Wheel Rack or Wheel Machine for information.
For more info on offset, see the Wheel Offset FAQ, which also has a fitment guide for 2G DSMs.