I've also noticed, that despite Blackman's reported interest in making fasteners invisible, there is a suspicious seam on the back of the later TNG uniforms which i always assumed to be a hidden zipper; and a darker seam on the DS9/Voyager uniforms on the front, aligned with the collar split, which could also be a candidate for a hidden zipper.
I can post screencaps later if needed.
Of course those were zippers. Where else would they go?
Re: TNG dress uniforms:
The first and second season versions were super long, too. Almost like dresses.
http://tng.trekcore.com/hd/albums/2x19/manhunt_hd_011.jpg
http://tng.trekcore.com/hd/albums/2x19/manhunt_hd_064.jpg
They got shortened for the 3rd-6th seasons, and again for the 7th season. The cut of the gold piping changed as well.
Season 4:
http://tng.trekcore.com/hd/albums/season-4/4x11/datas-day-hd-450.jpg
Season 5:
http://tng.trekcore.com/hd/albums/season-5/5x21/the-perfect-mate-hd-347.jpg
Season 6:
http://tng.trekcore.com/gallery/albums/s6/6x10/chainofcommandpartone068.jpg
Super shortened Season 7 version:
http://tng.trekcore.com/gallery/albums/s7/7x02/liasons012.jpg
Originally the pants for them were spandex, form fitting leggings almost (Lwaxana gets a gander at Picard's goods in "Manhunt"):
http://tng.trekcore.com/hd/albums/2x19/manhunt_hd_052.jpg
They switched to pants closer to what the standard uniforms used afterward.
Also important to note, but seemingly widely unknown -- this uniform (in it's last iteration before the Insurrection dress-whites) had a further modification, only barely visible in "You Are Cordially Invited...," adding the UFP logo belt to it that was later worn by various admirals in the FC-style uniforms. Voyager never utilized this modification, likely for the same reason they never switched to the FC-style uniforms.
You can (barely) see Bashir and O'Brien wearing the belts in this screenshot:
http://ds9.trekcore.com/gallery/albums/6x07/youarecordially264.jpg
Re: DS9 and Voyager Starfleet uniforms:
There were several versions of these as well. Originally we are led to believe they are jumpsuits/coveralls. The cut of the opening at the top changed as well, between seasons 2 and 3 of DS9. The season 1-2 version was tighter, and more pronounced, with the collar folding upward at its corners:
http://ds9.trekcore.com/gallery/albums/1x03/prologue008.jpg
http://ds9.trekcore.com/gallery/albums/1x03/prologue047.jpg
http://ds9.trekcore.com/gallery/albums/1x03/prologue164.jpg
http://ds9.trekcore.com/gallery/albums/1x03/prologue124.jpg
http://ds9.trekcore.com/gallery/albums/1x19/duet001.jpg
http://ds9.trekcore.com/gallery/albums/1x16/ifwisheswerehorses020.jpg
The season 3-5 versions were looser, for lack of a better term and seemingly more comfortable:
http://ds9.trekcore.com/gallery/albums/3x01/thesearch1_142.jpg
http://ds9.trekcore.com/gallery/albums/3x01/thesearch1_010.jpg
http://ds9.trekcore.com/gallery/albums/3x10/fascination_138.jpg
http://ds9.trekcore.com/gallery/albums/3x10/fascination_025.jpg
Note also, that the color of the undershirt/tee changes between seasons 2 and 3 from a lighter gray to (in some cases) a more mauve/lavender. This could also be due in some small way to lighting, I guess, but also clearly they switched the shirts as well.
Picard got a specially designed version for Generations, including (for some reason) a belt:
http://movies.trekcore.com/gallery/displayimage.php?pid=56281&fullsize=1
http://movies.trekcore.com/gallery/displayimage.php?pid=56290&fullsize=1
By the time Voyager came around, the undershirts changed color again, and even the cut of the coveralls changed, and Janeway's pips in particular fasten to her collar in a much different way than they do with Picard, Sisko or any of the other Starfleet personnel, and the shoulder pads for all are much more pronounced.
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0099.jpg
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0177.jpg
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0137.jpg
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0209.jpg
Also, for some reason, Tuvok, Kim and the Doctor don't leave their uniform coveralls unzipped all the way, as we'd seen Sisko, Bashir, Dax, and O'Brien (and all other Starfleet personnel do previously):
http://voy.trekcore.com/gallery/albums/3x19/rise017.jpg
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0208.jpg
http://voy.trekcore.com/gallery/albums/1x01a/caretaker_0416.jpg
Later in the series, we learn that the uniform is actually (supposedly?) a jacket and pants, not coveralls (or both):
http://voy.trekcore.com/gallery/albums/5x13/gravity_099.jpg
Also revealed was that the undershirt/tee has a zip up the front collar:
http://voy.trekcore.com/gallery/albums/5x13/gravity_164.jpg
And that there are alternate undershirts that can be worn:
http://voy.trekcore.com/gallery/albums/5x13/gravity_181.jpg
http://voy.trekcore.com/gallery/albums/5x13/gravity_357.jpg
http://voy.trekcore.com/gallery/albums/3x12/macrocosm_137.jpg
http://voy.trekcore.com/gallery/albums/3x12/macrocosm_143.jpg