æ is not always a dipthong, depends on the language in which it is being used, in old english it represents a specific vowel that is "between" a and e, not a dipthong that merges them.
this vowel is called "ash" in english (in reference to the tree, not the sound of the vowel which in modern english sounds like ee)
in classical latin usage it IS a dipthong pronounced the way a long I is pronounced in modern English aaah-eeee (or more accurately [ai], sounds like 'eye')
I've been studying the IPA for awhile now (working as a classical vocalist), went and looked it up in my books.
EDIT - wanted to add that the greek and latin usages are the same. The latin lifted it from the greek (IIRC, it may be the other way around).
