The short answer is no.  This has been a topic of eternal discussion
for the last couple of years thanks to Volvo's insistence on developing
a completely proprietary car audio system with no apparent attempts to
encourage development of third-party solutions.  If you wander over to
the swedespeed forums (www.swedespeed.com) you will find no end of
discussion on this topic.  The general concensus is an FM transmitter
(I use the Monster idrive plus - the sound quality is awful and the
reception is average, especially as I live in the NY area and there
isn't really a clear frequency).  At Mobile electronics forum
(http://www.me-mag.com/) one of the installers has posted instructions
on how to get into the antenna in the back and hook up an in-line FM
modulator - as there are no antenna adaptors on the market for the XC90
(many advertise that they work but they don't;don't believe what they
say, check with the manufacturer, if they include 2006 with previous
years it probably doesn't work) - I am not going to do this with my
lease vehicle, especially as this is only to wire an in-line FM
modulator!  Finally, there are a number of companies advertising
in-line adaptors, usually hooked up to the CD changer controller.  This
works on older XC90s and other models, most often the 60.  This do not
work on the 2006 XC90.  Many of these same companies are advertising
that they have an adaptor coming soon for the XC90, all of these have
been listed this way for over a year.  Plus, depending on how bad you
want to do this, these will probably cost $300+.  You can replace the
entire stereo - if you own the car this is probably the best option -
keep the existing speakers and replace the head unit.  Other than that
a letter writing campaign to volvo is probably in order.