The BBFC has published a list of the risqué content in GTA IV on its website.
Apparently you'll be able to pick up prostitutes and choose from three levels of "service". "What follows is an un-detailed portrayal of masturbation, fellatio and intercourse," the BBFC reports.
"The character can also visit lap dancing clubs and request a private dance." However, "While the game contains sexualised dancing and the portrayal of sex, there is no sexualised nudity."
Elsewhere, all of the major attractions are in: guns knives grenades blood po-po cocaine. Read the idocument for the full rundown - be warned though, some might consider it to be a bit spoilery.
The BBFC has granted GTA IV an 18 certificate. The game is out on April 29th on Xbox 360 and PS3.
UPDATE: Since this story was published, the document has been removed from the BBFC website - but you can read the text elsewhere on Eurogamer.