Your local wallet's addresses can be explored on http://blockexplorer.com/ just type one of the addresses you recently sent coins to in the box and click on the addresses in the table to explore.
This is true, but it doesn't change anything. You can search your own addresses and see what you paid for because you have the secret knowledge necessary to make sense of that info because you were the one who did it. Give those same addresses to your cop buddy, and send him to the blockexplorer site, and ask him if he can figure out what you bought or from whom without your help.
Silk Road takes deposits on their site, which winds up being a concentration point of illegal activity. It will show up quite clearly on a heat map, and could potentially be identified by depositing and withdrawing a certain amount of tracked coins, at certain intervals. A tech-savvy cop buddy could observe (a) bitcoin-like network activity at Bob's house, and (b) a flow towards the Silk Road "concentration" in the block chain. Observe several data points over time, and you can build a decent body of data evidence. Or simply observe Bob's web browser and BlahBlah Tor Site generating network activity at similar moments.
Serious anonymity takes a lot of work, and is darned near impossible if you are making a "noticeable" impact on the network, with your activity.
New users should get into bitcoin with their eyes open. Various amounts of tracking are already done with cash-purchased prepaid phones and debit cards. If it's an electronic record, it can be data mined for patterns of money flows. The government is quite skilled in picking signal out of noise, even encrypted noise.