I am also confused..
CCIE R&S Guide (3rd Edition ) says that ,"Cisco Switches default to use DTP desirable mode". However no specific model is mentioned here.
3560 software configuration guide says that, default mode is "dynamic auto"
CCNP BCMSN Official Exam Certification Guide 642-812, 4th Edition (Page-125) says that the default mode is, "dynamic desirable"
ICND2 Official Exam Certification Guide 2nd Edition 2007 (Page-30 ), says that "2960 switches default to an administrative mode of dynamic auto".
For 3550 model, the link
"The default switch-port mode for all Ethernet interfaces is dynamic desirable."
So...what now ??
The "older" switches wanted to form trunks, hence the dynamic desirable was the default.
Newer switches are a tad more conservative, so the dynamic auto is now the default.
Where does the exam fall into this changeout? Unfortunately, I haven't got a clue. I would likely go with the newer "dynamic auto" answer unless proven otherwise. It may also be worthwhile to open a case with CertSupport folks about that. While they will likely not tell you what the answer is (would take the fun away!), they would pass the information along to exam content folks and they could be sure of updates/corrections as needed.
Cert Support folks are not necessarily technically savvy. So be professional, and give quotes/links to the docs and courseware.
My two cents.