Although they've merged as an organisation, their exams are quite separate still (although you can play pieces from the other boards syllabus in equivalent grades!). I have a feeling it's 2007 that they will be using one syllabus and there will be unified exams. I know that they are going to use the Trinity marking scheme

I much prefer the Guildhall criteria personally.
Here we are I found this on their site!
"1st January 2007
The important date to remember for music examinations is 1st January 2007. This is the date by which we will have a unified entry process, combined syllabuses, a single assessment procedure and a single certificate. All new music syllabuses and publications will be available from the summer of 2006 so that teachers have plenty of time to prepare their students.
Current syllabus validity
All current Trinity and Guildhall GRADE syllabuses, regardless of the end date shown on the cover, are now valid until the end of 2006. Trinity syllabuses for Piano, Keyboard and Woodwind which were due to finish at the end of 2005 are extended for a further year to the end of 2006
Interchange of repertoire
From September 2004 candidates for Trinity and Guildhall grade exams will be allowed to select repertoire from either syllabus. For example, a Grade 5 piano Trinity candidate could select one or more of their pieces from the Guildhall syllabus, or vice versa. "
Hope all has become clear!