As soon as I use any of my I2P-bitcoins outside of I2P, I'm outed.
And anyone who wants to buy things from me would have a paper-trail linking their purchase from me to their regular account.
Someone might be able to see that you have some bitcoins, but how would they know how you obtained them? Eventually, as a sufficient number of people start using bitcoin, trades will stay within the bitcoin system, i.e., there won't be much of a need to do exchanges. There will also be services like the Bitcoin Laundry (
http://bitcoinlaundry.com/).