Screenshot by Lance Whitney/CNET

BlackBerry's BBM messenger service finally hits Apple's iOS and Android, but there's a catch.

The app is set to go live in the next hour, according to BlackBerry's blog. BBM fans who signed up on the official site will be able to use the application immediately. But those who haven't can download the app to "hold your spot in line," and will get an e-mail when they can start using the service.

(Updated at 11:54 a.m PT: the app is available now on Google Play and Apple's App Store.)

BlackBerry said 6 million people signed up for information on BBM.com.

"If you didn't sign up in advance, don't worry -- we are focused on moving millions of customers through the line as fast as possible," the blog said.

Screenshot by Roger Cheng/CNET

Even as BlackBerry institutes a phased roll out, the process of signing on still hasn't been smooth. Some people have encountered problems setting up the service, with the problem attributed to "temporary server error."

BBM was supposed to launch last month, but was pulled after a leaked copy was distributed and performed terribly in the wild. BlackBerry said the unintended launch caused issues that the company needed to resolve.

The fervent demand for the app is a counterpoint to the common thinking that BlackBerry isn't long for this world. While sales of its newer BlackBerry 10 devices are in retreat and the company has whittled its focus down to the big businesses and governments, BBM persists as a popular messaging platform.

BBM continues to be a popular platform around the world, particularly in emerging markets where cheap, older BlackBerrys are still in demand.

Still, the delayed service for many casual users interested in BBM may turn off some users who had abandoned BlackBerry to go to Apple or Android, but were interested in reconnecting with BBM friends.