Language / Data model / Objects, values and types (2.6-3.1)
third paragraph, has the following note:

"(Implementation note: the current implementation uses a
reference-counting scheme with (optional) delayed detection of
cyclically linked garbage, which collects most objects as soon as they
become unreachable, but is not guaranteed to collect garbage containing
circular references. See the documentation of the gc module for
information on controlling the collection of cyclic garbage.)"

There are two issues here: there are now other implementations that act
differently; CPython may change (as GvR reiterated strongly today in the
PyDev thread "__del__ and tp_dealloc in the IO lib", in response to a
request that the current behavior be defined and documented as not
permanent).  So I suggest:
change 'the current implementation' to 'CPython currently'
add "Other implementations act differently and CPython may change." (at
the end)