Oooh...sim card probs...there is a thread(s) dedicated for n900 sim card probs...it prob. is (hopefully... ) just that it isn't sitting correctly and connecting properly...there are a variety of fixes ...gimme a sec. I'll check the n900 device sub forum...and hit you with a link so you can read through and at least post there ...if not find a suitable fix...

What exactly did you put in the search box? This is such a common error and there are so many threads about it that I am not even going to bother giving you links as I would fill a page with them.

The problem is the loss of contact to the SIM card or to the modem chip, caused by bad soldering.

The most common solutions, in the order of difficulty, starting with the easiest:

1. A piece of folded paper under the battery to push on the components and renew the contact.
2. Hammering down the SIM card contacts (may not work if the problem is the modem chip).
3. Metal springs glued to the chips.
4. Reflowing the solder in the oven.

Hmm
it could be a few things...
the card may not be seated properly...you say it connects and then doesn't ...and then does...that may be as simple as a piece of thin folded paper placed over the sim as many have done...in a variety of ways..to add enough pressure to enable the sim to contact the pins properly...
Hmm..could be something as rudimentary as that...a physical fix.
or it may be internal....
or it could be the service area...
anyway...Best suggestion is to definitely read ...esp. read everything you can on the topic and process of elimination utilizing the easiest and least invasive methods 1st and narrow down the list of probable causes and apply the fix that works. You know best how and what your device is performing ...better than I or any one else....

I'll post in this particular post a few...(patience..it may take a bit)...threads where you may find the particular answers to your dilemma ...and/ or you can post probs..questions and screams for help
I really suggest you use sub-forums to post in...if you are asking what everyone knows is a common question...they'll direct you to the section where that issue lies....considering you are asking about the n900....the subforum and freemantle forum are the places to shout out.
Asking a specific question in general forums isn't a grand idea...since your question will very quickly get buried and lost ...in the place no one would look for such a question..And remember...if in doubt...and you can't get an answer out of the search engine......google is your friend....
Anyway here is a thread concerning the sim card and common probs not being recognized.

http://talk.maemo.org/showthread.php?t=36447

Haha...pichlo beat me to it...
And ya he is right...there are a lot of threads on the topic....google....man...
type...something like ...
nokia n900 sim card ...problems...(or) ...not recognized...(or)...cuts out...
and tons of maemo-site links will be at your disposal.
It is wording specific in alot of cases.
Anyway...Pichlo is dead on...it tends to be the common...connection issue resolved with artificially applied pressure...ie piece of paper...that tends to fix that common prob.
Anyway....I gotta hit the hay...tell us how it worked out though Abdel...ok?
Nitey-Nite

nice guys it works since i applied it no disconnection again. Ok i'll hence forth use google my good friends. Thanks for your helps once more.

Well hey!
Great!...
The (guessed at) prob was correct!
Well the odds were high in it's favour...
But don't always expect such good luck in puzzling out probs.
Some take quite a bit of time to diagnose properly...and to iron out ...if you get them. Just remember to read and bookmark alot for reference...never just accept the 1st answer either....read every possible answer on that particular topic 1st and then make an informed decision. Since over time ...solutions change.
And ya....you got it...try (differently) asking the same question of the search tool....and easiest is to first go into the sub forum section that your answer most likely exists in ...and then use the "search this sub-forum" search tool bar button just barely above the posts.... that will elicit more refined responses...
and of course....when in doubt ...Gargle it.
Later Gater

You are great @endsormeans

Hey now!
No need for name calling!
oh wait a sec...you said "great" ...
oh...well...hummm...
if that's the case ...
I do answer to the lowly title of being referred to as ..
"That-Incredibly-Ingenious....Epitome-of Awesomeness...Guy"...
It used to be longer...
but I discovered it may be a tad too vain.
So I shortened it by a few paragraphs.
But hey what's in an (obvious) title eh?
hehee
s'all good man..
Just "endsormeans" or "paul" is fine..
"great" is too "great" for me...
glad everything worked out...
and truth of the matter is...
you figured it out...not me
So...uuuh-huh!
who is really great here?
Not me man!
It's all you

See ya Abdel

Hav'a'gud'un
(canadian slang for "Have a good one")

Goodmorning maemo users i am sorry if i'm disturbing by posting here again but help me to find a thread that speaks about camera problem. My camera application when launched capture images without saving and when i try to capture 2 or 3 times, it will freeze on the last picture i took and whenever i launch it, it comes with that same photo except if i reboot. May God rewards you manifold for making those who love maemo to really enjoy it. Thanks once more.

Are u using camera ui-2 aka the one that comes with cssu ?