Scholars are divided on this issue, as smoking did not exist in the early days of Islam.
While some consider smoking to be makruh (reprehensible but not considered as a sin), more and more scholars are adopting the viewpoint that it is actually haram (sinful). While neither the Quran or the Hadith explicitly states flat-out that smoking is either makruh or haram, there are many general guidelines which point towards it. Among others:
1. Knowingly committing acts that harm yourself is forbidden. This comes from two verses in the Quran:
"And spend of your substance in the cause of Allah,
and make not your own hands contribute to (your) destruction; but do good; for Allah loveth those who do good."
[Surah al-Baqarah (The Cow) 2:195];
"O ye who believe! Eat not up your property among yourselves in vanities: But let there be amongst you Traffic and trade by mutual good-will:
Nor kill (or destroy) yourselves: for verily Allah hath been to you Most Merciful!"
[Surah al-Nisaa (Women) 4:29]
2. Danger and inconvenience to others
To knowingly cause danger and inconvenience to others is considered a sin. That can arguably include second-hand smoking as well.
"And those who annoy believing men and women undeservedly, bear (on themselves) a calumny and a glaring sin."
[Surat al-Ahzaab (The Coalition) 33:58]
3. Addiction
Addiction itself is not a big issue if it does not interfere with one's religious duties. In the case of smoking, though, interference with religious duties can be a major issue. One major example would be fasting.
Smoking is not allowed during the daylight fasting hours of Ramadan, for it involves intentionally introducing foreign substances into the body.
4. Noxious Smell
We need to go into the hadith for this. Muslims are discouraged (though not prevented) from consuming foodstuffs such as raw onions that give off a pungent odour. By inference, the noxious smell that smoking brings about would be included as well. One authentic hadith states:
“Whoever has eaten from such greens as garlic, onions or leek should keep away from our mosque.”
5. Waste of Money
"And render to the kindred their due rights, as (also) to those in want, and to the wayfarer: But squander not (your wealth) in the manner of a spendthrift.
Verily spendthrifts are brothers of the Evil Ones; and the Evil One is to his Lord (himself) ungrateful."
[Surah Al-Israa' (The Night Journey), 17:26-27]
The wasteful expenditure of resources is frowned upon in Islam. Expenditure on cigarettes can run to a large amount of money due to the high cost of cigarettes brought about by taxation and import duties.
This waste of resources does not only involve the cost on the smokers themselves; it also involves the cost to society as a whole in terms of lost productivity, increased healthcare expenditure, and the like.
A more detailed article on this issue can be found here:
http://islam.about.com/library/weekly/aa090600a.htmSo bottom line... there is nothing 'hip' about smoking when all indications are that it is at the very least frowned upon in Islam.
And if you really want to join Islam just so that you can smoke... you're making a big, big mistake.