I pronounce it (ih-serbius), for several different reasons..
Y -> I is a linguistics trend in many languages, dead and alive
cf. english (where Y and I have been mostly merged), welsh (still using Y, cf. dafydd for david) and in many other languages (notably the transcription of the play 'Tristan u Yseult' as 'Tristan and Isolde')
The name Yttrium, an element, is pronounced as "Ittrium", not "Yuhttrium".
Also, Y tends to be pronounced as a vowel (I) when in front of a consonant, and as "Yuh" (a consonantal sound) when in front of a vowel.
Yesterday, Yeast, Yes, etc... as a consonant
Yserbius, Ybarra, Yttrium, Yseult, etc... as a vowel, short I or long E.
obtw. Ybarra is pronounced "e-(B)AR'-ra" with a long E and a rolled r, and a b somewhere between B and V (the spanish "be grande".)
Iours truli,
SirZantac