Will this grow into a tree? In a few hundred years?
SOLVED: Eucalyptus - Species?
Yes it looks very much like a Eucalyptus but I doubt you'll have to wait long before you're chopping it back!
I had it once but it died to spider mites. I think it's silver dollar eucalyptus - E. cinerea? http://www.australiaplants.com/Eucalyptus_cinerea_foliage.jpg