I have a Nokia 6100, I use it to browse WAP pages, like all browsers
its build-in browser has a cache to store visited pages, serveral
months ago the caching mechanism went wrong, when I visit a previously
visited page, it shows the cached content, without connecting and
checking for updated page, I have to choose 'download again' every
time to get the updated page.
I tried upgrading the software to the lateset V05.51 version, but
didn't solve this. Any ideas?

Most cell phone manufacturers (Nokia included) have a one year
standard warranty in case of phone malfunction... which yours
obviously has.  I would contact your cellular service provider (i.e.
Cingular, AT&T) and ask them how to go about a warranty exchange on
your phone providing it is under a year old.  If it is over a year
old... you may inquire about upgrading to another phone or new 6100 or
purchase phone insurance through your carrier and file a claim when it
goes into effect.

Almost forgot another option... visit your local retail store for your
service provider and inquire about sending the phone into Nokia for
repair and obtaining a "loaner" phone while it is away.  Since the
6100 operates on a SIM card you can keep your card and store your
phone book so you won't have to worry about transferring that to a new
phone.