I am a big fan of retractable cables and my existing USB-to-microUSB sync cable didn't work. So I have found I perfect solution for me - dual-head sync+charge retractable cable (black head for sync/charge, red is modified for charge only):

http://www.seidioonline.com/product-p/cbuymcr2.htm

Just verified it works with N900, although they say only works with Palms, Blackberries and Droids - I guess it just follows the spec...

May not be the cheapest solution, but universal - my BT headset does not require microUSB cable mods, so I now can charge N900 and BT from a battery backup using the single cable!