depends. most of the standard formulas for bike size -- inseam x 0.65, or inseam x 0.667 -- calculate bike size measured from the centre of the bb to the centre of the top tube. this is the traditional way to measure bike size. the problem with this is:

1. not all bikes are measured c-c. treks measure from the centre of the bb to the top of the collar, meaning that a 56 cm trek is actually closer to a 54 cm in traditional sizing.

2. the most important metric of bike size is top tube length, assuming that you have enough standover height. that means that, if you have a long torso, you'd probably fit better on a size up in a trek [for example]. a 54 cm trek has a 54.6 cm tt, but you might be more comfortable on a 56 cm trek [remember, this would be a 54 cm bike measured c-c] with a 56.2 cm top tube. some manufacturers build with longer top tube, though; a m/l litespeed siena, for example [compact frame with a virtual 55 cm size] has a 56 cm top tube, and my 50 cm marinoni [measured c-c] has a 53.5 cm top tube.

3. compact frames have relatively more standover height than traditional geometry frames, so you could straddle a larger compact frame without sacking yourself if you need the extra top tube length.

my own experience is this... i have been riding my 50 cm marinoni for several years. according to the standard formula, my size would be about 51 cm. however, by trek's measuriung standard, it would be a 52.5 or 53 cm bike [measured to the top of the collar], and i am very comfortable [counter-intuitively] on my 52 cm kona cyclo-cross bike. however, i've always found the 53.5 cm top tube a bit short, even with a 11 cm stem.

this year, i got a litespeed teramo in size m, a virtual 53 cm compact frame. it fits absolutely perfectly with a 54 cm toptube and 10 cm stem. the headtube length is identical to my marinoni, but the headtube and seattube angles are slightly slacker. the fit is so much better -- even though i was always comfortable on the marinoni -- that most of my riding buddies have commented on how much better it is when they see me ride.

so the answer to your question really is "it depends." if you ride a 54 cm bike, measured c-c, with a relatively long top tube [say, 55.5 cm] and a long stem [say 12 cm] and feel comfortable, then there's no reason why you couldn't fit on a 57 cm frame [compact or measured centre-collar] with a 57.5m toptube and a 10 cm stem, assuming that you have sufficient standover.

the important thing is to compare apple-to-apples, lengths-to-lengths. take a tape measure to the bikes.