Is geliyom just a shorthand for "geliyorum" or does it have some other meaning?

 Yes. "Geliyom" is a shortened for "geliyorum".

In daily life some Turks shorten some verbs as its easy way to communicate !!!

like you brits shorten some words like " in it ?" instead of  " isn"t it "?

Thanks - but this brit doesn´t shorten things like that......
"isn´t it" is already a recognised shortened form of "is it not"

Yes it is. In fact all the grammatical persons can be shortened similarly in colloquial language.

Geliyorum > geliyom (I´m coming)

Geliyorsun > geliyon (You´re coming)

Geliyor > geliyo  (He/She/It´s coming)

Geliyoruz > geliyoz (We´re coming)

Geliyorsunuz > geliyonuz (You´re coming)

Geliyorlar > geliyolar (They´re coming)

With question particle mi, order of suffixes may change (it always comes as the final suffix with shorthand version):

Geliyor muyum? > geliyom mu? (Am I coming?)

Geliyor musun? > geliyon mu? (Are you coming)

Geliyor mu? > geliyo  mu? (Is he/she/it coming)

Geliyor muyuz? > geliyoz mu? (Are we coming)

Geliyor musunuz? > geliyonuz mu? (Are you coming)

Geliyorlar mı? > geliyolar mı? (Are they coming)

Of course during formal speech you should use the longer form.

 yeah  and you Turks say di mi ( di)

We shorten every possible word/sentence.

bi´ dâ´ki sefere hatırlı´ca´m ( bir dahaki sefere hatırlayacağım)

 Is "ain´t it?" more to your liking? I am from South London, after all

I dont think its write to compare "geliyom" with "isn´ t it" . Its more like "ya" instead of "you".

 Dear Zeytinne, we are talking about shortening the words,any words. Therefore any word that has been shortened is our subject. isn"t it or geliyom..

"I dunno what ya all talkin about" gibi bi´şey  

 yep u"re rite man ! ..) the subject is gettin worse in it ?..)

Twas no my fault, but geliyom´s  

What I wanted to say is that if I use "isn´ t it" or "don´ t" insead of "is not it" or "do not " is right...but if i use "geliyom" instead of "geliyorum"... this sounds like getto talking

Even if not proper, geliyom is still acceptable compared to geliom. 

I think "geliom" is only seen in written language of some people. It´s not somethng you may heard in daily language.

 I agree that "geliom" is seen in written language like in chat rooms by people who are lazy to write the whole word "geliyorum". But you can hear "geliom" in daily life used by again lazy people. It is also up to that person"s dialect and education status. Sometimes people uses extra shorthening words deliberately just to be looked funny. Like "seviom" =seviyorum ,"içacam"="içeceğim" . These sort of usages are giving Turkish nothing but harm.

 "doubt "

Come on si++, you don´t have to be didactic all the time. Why don´t you just let people have a bit of fun?