What exactly does a CSS filter do? I've never heard of them... <added>If they are the MS-proprietary ActiveX browser add-ons I just read about in webmonkey, circa IE 4.x, I wouldn't count on them being very useful...</added>
I've had no problems with making CSS positioning and font control cross-compatible (with the notable exception of NN4.x), but that's all CCS1. I don't think CSS2 is very reliable yet.
CSS "filters" are indeed M$IE propriety properties e.g. alpha, etc. They will not render in other browsers OR in IE6 when rendering in STANDARDS mode. For the most part, they are only supported by IE5.5 - the W3C does not endorse the M$IE propriety "css filters," and it is doubtful if it ever will.