It should be okay though. When its plugged in it'll be recharging yeah. When it has finished charging the battery, I'd guess it would switch off the batteries source to the power... not sure on that, but it should.
The only reason it would shorten battery life is if your continually charging and discharging it when it hasn't fully gone down. Lithium Ion corrects this though, and I think it takes out the memory some batteries have so that it doesn't bother it as much as older style batteries do.
Of course, charging it will always decrease the life of the battery a bit but this is normal. A good battery can last about 3-5 years of charging use. You can then just replace the battery. If you are worried about that though, while it is plugged in, you can take out the battery, though if you have been using it outside or whatever, of course it wouldn't charge it. This would atleast stop charging when you don't think it needs it since it hasn't dropped down enough
Hope this helps