what the question means is if i have an "average" frame with a 68 mm square taper or isis cartridge in, would that happily take an external bb?
or to put it another way, is the bb tube different on "new bikes" which are bulit with externals as appose to "old" bike using a cartridge??

yes external bb will fit any frame as long as its mtb sized bb not bmx or something funky like that tho im sure you can geta specific ones for larger bbs
and length wise they come with spacers so you can fit to any one

No, it will be fine. With the BB cups you'll get a set of spacers which you add to the drive and non-drive sides as needed / instructed by the leaflet inside. This is what adjusts the BB to fit the 68/73/e-type/chain device variabilities.

[edit: I mean yes, it will be fine / no there are no tube variations from old to new style. At least until push fit bearings in BB's become more commonplace….]

You'll also have to get the BB shell faced on both sides to ensure the external BB cups sit parallel to each other.