Surprisingly, quite a few people have done or are doing similar journeys by bicycle. All of the below have been in some way inspirational or useful to us, and are worth a look:

Cycle UK to China
Chris Smith, author of "Why don't you fly", cycled from London to Beijing in 2002 with a round tour of the Indian sub-continent thrown in just for fun.

Penny Farthing World Tour
Cycling around the world solo on a custom built Penny Farthing. And you thought we were mad...

Pedal to China
A cycle tour in 2004 from London to Beijing like ours, except via Russia rather than Iran. Would be a bit cold at the time of year we are going!

Cycle Home
Having been teaching in Beijing for a year, two recent graduates decide to come home to the UK on bicycles. These guys are setting off in August 2005 and are following a similar route, so we may run into them on the way!

Cycling to Sydney
Two girls cycling from London to Sydney, starting along a similar route to ours (as far as Iran). They set off in July 2005.

Gordon Goes Global 2005
One man cycling round the world on a fixed wheel bike. This is as masochistic as it gets!

Edward Genochio's trans-Asia bicycle expeditions
Various solo tours across Asia. Found fame in the UK press after a Mongolian horseman stole his bicycle!

Round the World by Bicycle
Round the world, and then some; 50,000 miles, more than three times the distance required by the Guinness book of records for a round the world cycle.

Charity Ride
London to the Himalayas. A very good source of advice for planning a cycle tour, written with the benefit of hindsight.

For people who think we are insane for wanting to cycle through Iran, here are some good accounts of past trips:

Tallabomba's Europe to Asia by Bike
Iran by bicycle
My Family's Tour of Middle East... by bike!
Tandem to Turkestan