On the hook Max 
You do realise what little it truely means in real world electronics though with supply noise, schott noise etc which totaly out weighs the significance of the LSB.......
actually it means a great deal....

you need to live in a world of ubergeek audio electronics mate.... there are mixing consoles and mic pre-amps out here with who's designers are aiming for noisefloors approaching that of a piece of bare wire...... 8-) (Re : Johnson noise)
real world figures for 24 bit systems using professional grade converters , are a typically of the order of 112-120dB (A-weighted) useable , linearly accurate Dynamic range, some claim better than that, ( but my god you really don't want to know the price !)
domestic/semi-pro is usually more like 101-110 dB (A-weighted)
SOME manufacturers put it a little differently, dCS for example... won't happily refer to a product as being capable of 24 bit resolution, unless the LSB is of some demonstrable use.....

and other foibles, like insisting on using the more technically accurate description of Sample rate " Kilocycles per second" as opposed to a KHz figure...
http://www.dcsltd.co.uk/technical_papers/bits.pdfothers have shown ways of dithering the audio data stream to allow perception of signals theoretically below that of the applicable system noise.. effectively, (according to some) getting 20 Bit performance from a 16 bit media. ( Apogee UV22 for example, Prism-Sound also have similar technology )
the use of balanced signal paths, proper power supply implementations, and all sorts of other silly tricks get thiese guys in to noise performance territories that domestic manufacturers don't even dream about....
bear in mind , for example that the Pro-audio market generally uses balanced connection protocols, and, further example, Schott noise is a "Common mode" form of noise, and therefore theoretically rejected at the receiver end of a balanced connection.
not to mention intelligent installation design ....

there's lots of interesting stuff being done at the far edges of the audio geek universe (and i'm NOT talking about bloody snake oil Hifi 'dangle berries' like Russ Andrews and that lot purvey... )