1) Example of using a Python function in the expression to convert field values to upper case

Expression:
(!TS_ZONE!).upper()

2) Example of accessing properties of feaure geometry to convert area from meters to feet. Ths provides easy acces to the area and length properties of a shape fie.

Expression:
MetersToFeet(float(!SHAPE.AREA!))

Code Block:
def MetersToFeet(area):
    return math.pow(3.2808, 2) * area

3) Example of reclassifying a range of values.

Expression:
Reclass(!WELL_YIELD!)

Code Block:
def Reclass(WellYield):
  if (WellYield >= 0 and WellYield <= 10):
    return 1
  elif (WellYield >10 and WellYield <= 20):
    return 2
  elif (WellYield >20 and WellYield <= 30):
    return 3
  elif (WellYield > 30):
    return 4    

4) The Python expression and code block used in Calculate Value to check the existence of a field.

Expression:
CheckField(r"%input_features%", "%field%")

Code Block:
import arcgisscripting
gp = arcgisscripting.create()

def CheckField(in_Table, in_Field):
    fields = gp.ListFields(in_Table, in_Field)
    if fields.Next():
        return 0
    return 1