It's still supported by SQL Server 2008, even though it's on the list of deprecated features. This means that nobody can tell (except maybe a few Mircosoft employees, but they won't) whether this feature will still be available in the next version of SQL Server (2011??), and whether some lower compatibility setting will be required by then.

In other words - start investigating your code now, and if you use this feature, plan to replace your code somewhere over the following years. There's no rush, but you shouldn't postpone it either. And you definitely should not use GROUP BY ALL in new work.