Thorny one - representing greek letters and formulae in an english page

One of our accessibility-course delegates has come up with something that has me stumped at this stage:  they want to represent greek letters on their page.  They currently use <FONT> to switch to the symbol font to do this, and another delegate argues for Gifs with ALT text.  My views are:

1) Font tags

Are wrong. 

2) gifs with ALT text:

Do not scale, but better for screen readers and older browsers

3) use a <SPAN> with locale change to greek

don't know if this would work, possibly more correct.  not sure about how most browsers will receive it.

4) Character entities :

Probably purest way, probably inaccessible to screen readers and older browsers


Does anybody have a view on this? or any decent resources I can review?

Regards,

Steve