I've had the good (or bad) fortune to never work on a data warehouse of under a terabyte. If you want something really big like, oh, Southwestern Bell has, you need DB2 or Teradata (SWB's choice).
I don't work extensively with Oracle, but in my limited experience, it runs out of horsepower somewhere near a terabyte. Maybe the newer parallelism features will fix this.
One session I really wanted to attend this year was Multi-Terabyte Data Warehouse and MySQL. Having seen the challenges involved in getting systems of that size to perform efficiently, and thus being a skeptic when someone claims they can do so, I'd really like to hear what he's got to say.