High-quality commuting bike with "women's" frame My wife wants a high-quality commuting bike (riding almost an hour each way five days a week). However, she wants it to be compatible with skirt-wearing and, as a result, refuses to have a frame with a normal top-bar. 

Is there even such a thing as a high-quality commuting frame with the low top-bar of a women's bike?
If you and she are willing to throw down on a custom, I can personally recommend Violet Crown Cycles. Otherwise, consider the Rivendell Betty Foy/Ivan Gomez or building up a Soma Buena Vista... but she might actually do fine picking up a mass-market step-through that fits well (Electra, Jamis, and Trek have models worth looking at) and upgrading the components as desired.

For a ride that long, I would actually recommend against a Dutch bike; something speedier would likely be better. Believe it or not, an upright posture is not necessarily ideal; I find that a more forward-leaned posture keeps long skirts from blowing back and up in headwinds. (There are other ways to deal with this. The headband-round-one-leg trick works.) Avoid crank-forward/"flat foot" designs for this reason.

I will disagree with Batman above regarding durability. Though I've seen that claim many a time, I have yet to see anyone with a horror story regarding a mixte or step-through frame falling apart or anything similarly catastrophic... or even bothersome, for that matter. I've gotten along just fine with both my step-throughs; if they flex more, I don't notice it.

My somewhat ornery but experience-driven suggestions for skirt-friendly bikes:


Fenders (minimally a rear fender)
Chain guard. Chains like to eat skirts, as well as deposit grime on them. (Consider a belt drive, rare though they are -- lack of grime is one of the things I dearly love about mine.)
Disc brakes and/or a skirt guard on the rear wheel. Rear rim brakes LOVE to eat skirts; I lost two dresses to my Electra Townie in my early days with it because of this. (Check Etsy for aftermarket fabric skirt guards.)
Internal gear hub if she likes really long, full skirts. A skirt-eating derailleur is a long shot, but depending on the hanger configuration, it could happen.
Decent carrying capacity. I don't mind carrying a messenger bag while in a dress, but she might, especially on that long a ride... and even I (fifteen minutes door to door) sometimes sling mine into a pannier.
If the bike she picks only has one set of braze-ons for water-bottle cages (which is common for step-throughs), I recommend adding a handlebar cage mount; these are easy to find aftermarket (e.g. this one by Velo Orange).


And for bike-friendly skirts and dresses:


Pencil skirts longer than just above the knee are Right Out. Forget it. She'll tear it to pieces trying to pedal, no matter how it's gored or slit.
Shorter skirts can work, but wear bike shorts underneath. (If she's going to do that, though, why not just wear the shorts and change into the skirt at work?)
Full skirts work fine if she avoids skirt-eating chains and rim brakes. (The Victorians did it -- so can we!)
Maxi-style straight dresses work fine if the material has some stretch, such that knees don't leave unsightly "dents" in it.
Camisoles, for any dress neckline that isn't reasonably close-fitting. If she prefers an upright ride, this isn't necessary.


Good luck! I'm not sure the perfect bike for this use-case exists in the mass market (that's why I went custom), but she ought to be able to find something that doesn't compromise too much.