The rules really don't say. Our group has always played that you can't do this. I could have sworn that I read somewhere that you could not cast spells while using your bardic ability...but I can't find it now...
I'd vote no.
{EDIT}
I gave up looking to easily...right there in the class description...
SRD:
While singing, the bard can fight but cannot cast spells, activate magic items by spell completion (such as scrolls), or activate magic items by magic word (such as wands).
It's under the section of Inspire Courage...