Working backwards, you want as few steps as possible between the speakers and the source.

I would recommend having a Mac mini, with a soundcard that has a coax s/pdif OUT attached via Firewire to the mini. You then connect the soundcard directly to the BL5s using a coax cable, skipping the wireless portion.
You can control playback using either Screen Sharing on your laptop or an iPod Touch configured as a remote with Remote Buddy installed on the Mac mini. (You can also attach the Mac mini to a screen or television).

To extend storage space, you can have the Mac mini connected to a stand-alone server offering expandable memory. Such as the ReadyNAS+.

You can, of course, also have a soundcard attached to the laptop, but then it's not very mobile. The advantage of using a soundcard, and going directly to the BL5s from that, is that you're not limited by the 16-bit/44.1kHz limitation of the Airport Express, and can play back higher resolution files. The interior DACs on the BL5s are limited to 24-bit/96kHz, but that's as much as you'll need.

There are lots of good soundcards on the market, your Apple dealer will be able to recommend some. Or else try chatting with a Pro Audio store that caters to musicians, sound engineers - and these cards are not expensive.

Here's an inexpensive one:

Do you really think a good soundcard would be necessary? After all, jitter isn't really an issue any more, in fact it hasn't been an issue with computer soundcards for a number of years.

The tricky bit is that the BL5s are coax s/pdif and 24-bit/96kHz -- while the Airport Express is 16-bit/44.1kHz and optical toslink s/pdif.

The AE clips the resolution, and the optical/coax requires a converter.

By using a soundcard that has a coax s/pdif OUT you eliminate the need for a converter, and make it possible to feed up to 24-bit/96kHz files to your BL5s without any resampling of resolution.
Of course, nothing wrong with going the conversion route originally outlined - except that if it's absolute high-end that's required, then I'd give it a rethink. (Particularly now that www.musicgiants.com, Linn records and others are offering high-resolution downloads.)

I see what you're saying, the only shortcoming at the moment is that the vast majority of source material at the moment is still 16 bit 44.1 KHz (i.e. CD based) and is still likely to be for the forseeable future (especially since the mass market is currently heading the other way toward mobile, compressed music).

Optical/electrical S/PDIF converters are not very expensive (say less than £30) and shouldn't affect the music in any way.

stax 

Here's some info on the Apple Express' capabilities, from someone who really checked it out.

From this discussion, and very recent: link

I found yesterday an interesting article, maybe somebody is interested on that information too: