you can change the colour of it ok i think, or set the thing to have a width of 0 so it doesn't show at all. i'm sorry to say that i use adobe golive to make my websites so i don't know the code for it.
my site has a bunch of frames, all with the borders set to no width and the code in the source reads as:
<frameset rows="35,403,*" border="0" framespacing="0" bordercolor="white" frameborder="no">
i understand part of it, i think the bits you want to alter are the border= setting, the 'framespacing', 'bordercolor' and 'frameborder' settings.