Something

import Blender
from Blender import *

def collectIpoNamesFromSelection(excludeList):
  selectedObjects = Object.GetSelected()
  ipoNameList = []
  for i in selectedObjects:
    if i.getIpo():
      ipoNameList.append(i.getIpo().name)
    if i.getType() == 'Armature':
      if i.getAction:
        armIpos = i.getAction().getAllChannelIpos().values()
        if armIpos:
          for j in armIpos:
            ipoNameList.append(j.name)
  return ipoNameList

def changeInterp(typeInt, *ipoNameList):
  for k in ipoNameList[0]:
    targetCurves = Ipo.Get(k).curves
    for l in targetCurves:
      l.interpolation = typeInt
  return
ipos = collectIpoNamesFromSelection(1)
print ipos
changeInterp(0, ipos)