Simply put, we'd feel a tablet interface would be the most practical that a touch version of GarageBand could get.  Seems as if we were wrong, according to Apple who released a version for iPhone and iPod Touch today.

Apple's music app has let anybody from experienced musicians to teens who think they can play an instrument create and record their own music ever since its iPad release in March of this year (much earlier on OS X). Now GarageBand 1.1 is out as a universal app with support for small iOS screens.

• Create custom chords for Smart Instruments
• Support for 3/4 and 6/8 time signatures
• Reset song key without transposing original recordings
• Transpose songs in semitones or full octaves
• Additional quantization options for recordings including, straight, triplet and swing
• New audio export quality settings for AAC and AIFF (Uncompressed)
• Arpeggiator available in Smart Keyboard
• Adjustable velocity settings for Touch Instruments
• Numerous enhancements, including automatic fade out and improved audio import options

It's £2.99 from the app store, available now.  But we're not sure that something like audio recording and mixing (even as simple as GarageBand) has a simple enough home on the 3.5" display of an iphone.