Judging by the amount of entries in the search log this is a much sought after lead!
Despite the ease of construction and low component count I've labelled this project as moderate (orange) because it uses a modular plug, therefore requiring the use of specialised crimping pliers.
I've redrawn a schematic that I found with colour for ease of circuit tracing and some modification.
The original showed power being supplied by the PC via the CTS and DSR lines on the serial port but some PCs and USB to Serial adaptors can't power the circuit in this way. The radios that use this lead (that I have found the schematics for) supply 8 - 9v at the mic socket and the current supplied should be sufficient for this circuit so I've modified the schematic to suit being powered in this way.
If you find that your radio cannot power the circuit in this way simply diisconnect the orange (in the diagram) lead from the 8P8C plug and connect it to a PP3 or similar (9v) battery positive terminal, the negative terminal of the battery being connected to ground.
For the construction you can use strip board, dead bug method or even produce your own PCB if you like. Just follow the schematic. Component layout is not really critical for this circuit, although beginners (and the more experienced) may have to double check polarity / orientation of components.
Once made up check your soldering for bridged connections and wires for stray whiskers. If your construction is small enough it may fit in the shroud of the 9 pin D socket, if not a small box with short leads connecting to the 9 pin socket will work just as well.
The 8P8C plug (P2) is shown with the connecting pins toward you and the locking tongue away from you. Pins 3 to 6 are not used. This plug should be mounted on a lead that is not too long (signal losses) but long enough to be easy to use, a metre or so should be fine.
Updated 24 Aug 2011
Some users of the IC-440 UHF CB have reported that the OPC-1122U (Icom USB lead) does not work with their radios. This is proving problematic to resolve at this time as some users report that all is working while others report nothing works and a third "group" have no problem when using one computer, only to have no joy when using another!
I do not know if the same problems will occur with the above interface as I do not have the radio to test it with (if someone wants to send me one email me for my address!!). When (if) I discover (or someone kindly informs me) the definitive answer I will update this page.
To use the interface with your radio you will need the software relevant to your radio. This is usually purchased from the manufacturer or a third party. There may be programs available from other sources, offering various features. Follow the instructios in your manual / software instructions or help file for connecting and data transfer.
If you need software for your radio but can't find any, take a look at: icomrecievers.narod.ru, there are lots of models listed and they're not all receivers (as the name would suggest).
I've been asked how to modify this cable to use it on a PC with a USB port and no serial port.
Short of purchasing or building a USB to Serial adaptor or purchasing an OPC-1122U (has a USB adaptor built in) I have no other remedy.
I can report that the USB to Serial adaptor using the Prolific Chipset works well with this although Windows 7 (64-bit) had a problem installing the adaptor until I visited the Prolific website and downloaded the latest drivers.