I've been reading "A Draft Test Protocol For Detecting Possible Biohazards In Martian Samples Returned To Earth" (PDF.) It's pretty interesting -- they're even considering the possibility that Martian life might not be carbon-based:

If there were a life-form on Mars based on other than carbon-containing molecules, the energies holding such molecules together would not be much different than those for proteins and polynucleotides. Hence, bond breakage by heat or gamma radiation should be similar for Earth and Mars life-forms, and sterilization conditions for Earth microorganisms should eradicate microorganisms of similar size from Mars.

Irrespective of the chemical basis of any life-form, a confidence level of sterilization can be provided with only two assumptions: 1) any reproducing life-form must be based on macromolecules (i.e., polymers) with interatomic covalent bonds (not crystal lattices), and 2) since all such bonds have similar strength, destroying these bonds destroys the life-form.

Through most of the document they don't refer so much to "life" as to the problem of "introducing a replicating entity of non-terrestrial origin into the biosphere." But they've got a pretty nifty definition of "life" in there too:

Evidence for only a subset of these properties in an extraterrestrial specimen might constitute a sign of life (e.g., evidence for a self-sustaining catalytic system). However, it is the presence and combination of all of these properties that define life as we know it.

• Life is catalytic
  • There should be significant deviations from what chemical kinetics predicts
  • Life modifies its environment
  • Life consumes energy
  • Life creates waste products
  • Life is exothermic
  • Life uses thermodynamic disequilibria to build and maintain other thermodynamic disequilibria (in open systems or within a "wall")
• Life is genetic
  • There will be some system for storing and propagating information
  • There will be molecular distributions with significant capacity for complexity
• Life replicates and evolves
  • There will be evidence for replication of structures and complexity
  • There may be evidence (structural & chemical) of evolution of form & function