From the DMG/Compendium, the important part for your question:
Forced Movement and Terrain
Difficult Terrain: Forced movement
isn’t hindered by difficult terrain.
Blocking Terrain: Forced movement
can’t move a creature through blocking
terrain (page 61). Every square along
the path must be a space the creature
could normally occupy.
Challenging Terrain: Forced movement
can make some powers more effective or
hinder them, depending on the specific
challenging terrain. The DM can
require the target of forced movement
to make a check as if it were moving
voluntarily across the terrain, with
the same consequence for failure.
Hindering Terrain: Forced movement can
force targets into hindering terrain.
Targets forced into hindering terrain
receive a saving throw immediately
before entering the unsafe square they
are forced into. Success leaves the
target prone at the edge of the square
before entering the unsafe square. If
the power that forced the target to
move allows the creature that used the
power to follow the target into the
square that the target would have
left, the creature can’t enter the
square where the target has fallen
prone. If forced movement pushes a
Large or larger creature over an edge,
the creature doesn’t fall until its
entire space is over the edge. On the
creature’s next turn, it must either
move to a space it can occupy or use a
move action to squeeze into the
smaller space at the edge of the
A DM can allow a power that pushes a
target more than 1 square to carry the
target completely over hindering
Published in Dungeon Master's Guide.
And from the DnD FAQ ( http://wizards.custhelp.com/cgi-bin/wizards.cfg/php/enduser/std_adp.php?p_faqid=1396 ):
Are zones that deal damage (like the Wizard power Stinking Cloud) considered ‘hindering terrain’? Can I make a save to fall prone and avoid being forced into one?
No, zones are not considered hindering terrain. Hindering terrain refers to more permanent features like pits, cliffs or pools of lava.
The resultant answer: For zones, base DnD rules do not give a saving throw to avoid being put into the fire. For pools of acid, you are given a saving throw to catch yourself before it and fall prone.