I have a shimano dynamo hub on the front of my bike.
Does it need any special maintenance, or maybe it needs just a little synthetic grease once a year, like a normal front hub?
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The common Shimano Dynamo hubs don't actually use sealed cartridge bearings, see the DH-3N71 and DH-3D72 techdocs. Shimano dynohubs do use good seals, however, and should hopefully be maintenance-free for many thousands of miles.
Alistair Spence has a good exploded view of an Alfine DH-S500, which is very similar to the DH-3D71. He also links there to a disassembly guide PDF, the original of which can be seen (minus images) in the internet archive. The steps are basically the same as a standard front hub, with the caveat that it is important to be careful to avoid breaking the axle wire.
The Shimano hub dynamo bearings on one side probably not so difficult and you can look at their drawing but on the other side it can be difficult. I haven't tried to open the other side but you can check this out: http://www.bikeforums.net/archive/index.php/t-700163.html