Can porcelain tiles be installed over ceramic tiles?
Although I would never do this myself, eHow has a step-by-step for this.
Keep in mind that doing this will add thickness to the floor, so transitions into other rooms might be tricky. According to This paper &quot;an abrupt increase in
height&quot; of 1/4&quot; (6mm) &quot;is considered to be a threshold level for consideration of trip hazard for a pedestrian&quot;.
If this is done in a bathroom, the toilet flange (and/or other fixtures) will likely have to be raised to compensate for the new floor height.

Tile Over Tile Floors
1 Inspect the tile floor and make sure there are no
loose tiles; the existing tile must be
in good shape for re-tiling.
2 Clean the tile thoroughly to remove
any wax or build-ups on the tile
floor. Scuff or sand the tile to make
the surface a little rough; this helps
the new tile bond to the old tile.
Rinse the floor after it has been
sanded to remove any dusty particles.
3 Apply a floor leveler to the whole
surface of the floor. If you have a
raised area, begin there and work your
way outwards in a circular motion. The
floor leveler will fill in previous
grout lines and make the floor level
for the tile over tile application.
Floor leveler is usually latex-based
cement.
4 Use a screed to level the floor; a
screed is usually a long and very
straight board or level that will move
the excess floor leveler to lower
levels and will help you level the
floor. Allow the floor leveler to cure
according to directions, usually 24
hours.
5 Buy thin-set mortar that has a latex
additive in it or replace the amount
of water with a latex additive. Latex
or epoxy type mortars are best for
installing tile over tile. Mix and
apply according to manufacturer's
directions.
6 Set the new tile in small enough
sections as the thin-set mortar
directions call for. After the whole
floor has been tiled, install the
grout according to instructions. Seal
the grout if directions call for it.
Tile Over Tile Walls
1 Clean the
existing tile thoroughly to remove any
wax or soap build-ups. Sand the
cleaned tiles to rough up the surface
for better adhesion. Rinse the sanded
tiles to remove excess dust from
sanding.
2 Use a latex or epoxy based thin-set
mortar to set tiles into. Apply to the
surface of the existing tiles
according to manufacturer directions,
setting the mortar base at a medium
depth.
3 Install the tile a small section at
a time, according to the thin-set
directions. Apply grout and sealer
according to directions.