When it's done.
Seriously. No one here knows about Apple's schedule for anything, one can only make conjectures ala
Mac Buyer's Guide: Know When to Buy Your Mac . Pro apps like Final Cut Studio have irregular and long delays between major releases, simply because they are not needed. FCP as it stands right now is pretty feature complete, albeit it could use better file management. Motion needs serious bugfixes and some UI improvements, but it's a solid piece of the suite as it is.
Traditionally you'll hear about any updates around NAB, which is held in April.
Here's the release schedule so far:
NAB 1999 -- Final Cut Pro
MWSF 2001 -- DVD Studio Pro
March 2001 -- Final Cut Pro 2
December 2001 -- Final Cut Pro 3
April 2002 -- Cinema Tools
NAB 2002 -- DVD Studio Pro 1.5
NAB 2003 -- Final Cut Pro 4, DVD Studio Pro 2, LiveType, Soundtrack, Compressor
NAB 2004 -- Final Cut Pro HD, DVD Studio Pro 3, Motion, LiveType 1.2, Soundtrack 1.2, Compressor 1.2
August 2004 -- Production Suite
NAB 2005 -- Final Cut Studio: Final Cut Pro 5, DVD Studio Pro 4, Motion 2, LiveType 2, Soundtrack Pro, Compressor 2
March 2006 -- Final Cut Studio 5.1 released as a Universal application.