Thanks da_willem for the email address (solutions@cambridge.org). I sent them an email asking for access, and they kindly replied with the following info, which I provide here for those interested:
Go to:
http://www.cambridge.org/catalogue/catalogue.asp?isbn=0521831431&ss=res
Click on "solutions". You will be prompted for a username and password. Enter the following:
Username: lecturer
Password: cambr1dge (note: spelt with a 1, not an i)
The solutions available so far are from chapter 1 up to and including chapter 16. They are available as downloadable PDF files. The rest, according to the correspondent, will be posted as soon as received from Zwiebach. Enjoy.
I had incredible luck today. I went to a second-hand book shop and I obtained Green et al.'s "Supertring Theory - Volume 1: Introduction" (CUP) and the 2-volume "Superstrings - The first 15 years of superstring theory" (World Scientific), edited by John Schwarz, all for just AUD$ 50 ! I think a (frustrated?) physicist recently dumped his books there.. lol