There are many areas on the web you should read about the web
and cell phones.
A good place to start is w3c.org
Some of the many differences:
Most phones do not have the display capablilty of your PC,
on avg, 110x180 pix compared to the avg PC screen res of 800x600 pix.
This res web standard for a PC really has moved to 1024x768 pix.
Phones do not have the "ram" memory capacity as your PC.
Some phones will not be able to decode java script.
Most will not be able to display objects that are "FLASH"
but there are specialty programs for flash available for phones
like "FlashLite".
You can expect they will be able to handle HTML/DHTML/XML.
If fact xml is prefered design language standard.
You must be careful of describing fonts or graphics that may not
be capable in a phone.
the iPhone is unique as it will be able to diplay most all pages in
a "full-screen" mode as it also has a nice zoom capability.
But sill it can not display normal "FLASH Media content"
To design pages for PC's and be able to view on cell phone as the
same is almost totally improbable without "trapping out" the cell
as it enters your site and directing it to pages built specificly
for phones and then these pages need to be as universal
as possible for the many display resolutions of the many phones.
You may "trap" by use of browser,os,platform identifiers.
Once identified, you may direct to specific pages for it.
Same with jave script as a few lines of jave code can seperate
the "haves" from the "have nots".
The .mobi (dot mobi) domain sites are domains specific to "cell phones"
It may be 5 - 8 years before most phones will be able to rome the
web as you do on your PC. The iPhone & High end phones like the Blackberry
and some others has brought this all a little closer.
You may try
www.time.mobi to have a look at the mobile format.
Look at it on your phone first then your PC to see the diff.
You might have a very good idea at this point.
But it is much work to to try to format to full capability for all
mobile phones unless you start from a minimum standard standpoint
or at say a minimum standard as the RaZR being the most popular format.
Writting a page for this would insure that 95% of phones could view
your content. But the fancy content would be more limited.
Hopefully this is a informative start but there is much to read and learn
on this subject if you really want to make your site mobi freindly.
Rather than downgrading you content for .com's & PC's
It is more flavorful to write for PC them trap the phones and write
specific content for them.
...magoo