Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
£10 a month

Which gets you:

-Unlimited Streaming at 320kbps Vorbis
-No ads
-Over 10 million tracks
-Online and offline

On:

Windows (XP, Vista, 7)
Mac OS X (10.4, 10.5, 10.6 including PowerPC)
Linux (Debian/Ubuntu or Wine)

iOS
Android
Symbian
Windows Mobile

You can use any number of the above clients (I use it with 6 devices on a regular basis). When I make a change to my playlists, that happens on the other computers/devices instantly.
 
So would this render Spotify pointless?

That would depend on:

-Pricing: Spotify has a Free option, a mid-range option that costs £4.99 ($8), and a premium option that costs £9.99 ($16)
-Quality of audio streams (iTunes currently offers 256kbps AAC downloads. Spotify Premium offers 320kbps Ogg Vorbis)
-Amount of music available on the service
-Platforms it works on (presumably it'll only be in iTunes 10+ on Windows or Mac OS X as well as iOS devices)
-Offline functionality
-Regions it is available in
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.