Whee! I take, two steps forward (I take two steps back!)...!
From the Main FAQ... but does it still apply?
Will extradimensional items rupture a bag of holding?
The DUNGEON MASTER’s Guide says that a bag of holding
placed within a portable hole tears a rift to the Astral Plane.
Bag and hole alike are then sucked into the void and
forever lost. The DUNGEON MASTER’s Guide also says that
when a portable hole is placed within a bag of holding, it
opens a gate to the Astral Plane. The hole, the bag, and any
creatures within a 10-foot radius are drawn there,
destroying the portable hole and bag of holding in the
process. However, the description for Heward’s handy
haversack makes no mention of any rifts or gates. This
implies that only the combination of a bag of holding and
portable hole forms a rift or gate. Thus, a bag of holding
could be placed inside another bag of holding with no
unusual effects.
It’s a general rule that you can’t mix items containing
nondimensional or extradimensional spaces (things that are
bigger inside than out) with each other or with portable holes.
Such combinations tend to strain the fabric of the cosmos.
Putting one bag of holding within another is just like putting
the bag into a portable hole. Items that function like bags of
holding, such as Heward’s handy haversacks, cause the same
mishaps when mishandled.
Note you can freely go plane hopping with portable holes,
bags of holding, and the like. Spells that produce their own
extradimensional spaces, such as rope trick, pose no danger to
occupants who may be using portable holes, bags of holding, and the like.
-Hyp.