I have XM in my car and love it. I have reservations about listening to head phones while riding on the road. Not hearing other traffic could be a problem. XM must have a direct line of sight to the sky. When I drive under much of anything the signal interupts. So riding in the woods might be a problem with trees overhead if they were very dense.
Raliegh C40 Hybrid Bike. Cannondale R900 Triple. Schwinn World - Fixie Conversion
A friend of mine has the portable XM receiver but I don't think he's tried out that aspect of it yet. I think he's just used the home base unit. If I'm not mistaken, his came with a smaller antenna that you wear around your arm. Also I should mention, that his portable receiver cost close to $300. However that model allows you to save something like five hours worth of music on it. I'd love to have one.
I've got XM at home and I love it. It was a reward to myself for going so long without TV. Now I can listen to CSPAN and it's just as exciting!
Any of the XM2Go units will work - Pioneer just came out with a new unit, in addition to the original Delphi MyFi and Tao unit. They will store up to 5 hours of programming too, so if you're out of range of the signal, you still have something to listen to. The signal is pretty robust, so unless the tree cover is very heavy, you should be good to go. Also, many urban areas have repeaters that will give you a much stronger signal that is beamed from land-based antenna.
I have a Roady and a Roady2 and use both on my bike. You'll need headphones with volume control. Also www.xm411.com has a store where they sell some stuff to make your unit portable - they are their exclusive designs. I have an Armie. It holds the Roady, batteries and antenna in an armband unit. He also sells a fanny pack carrying case. I put the Armie on my handlebars or in my jersey pocket when riding my bike and on my arm when mowing the grass. With the lights turned off the batteries last several hours. Turn off the lights by hitting 2 3 2 and pushing in the round scroll button. You can also make a battery pack from Radio Shack supplies. You need to buy a unit for 8 AA bats and jump the last two slots to make it function with 6, unless you can find a 6 unit pack. I couldn't and didn't want to wait. I use 15 minute rechargeable rayovacs. Using 6 regular batteries may be too much juice. XM411 also sells battery packs and rechargers.