Of course Christians can be circumcised, I don't know what ad_o is talking about unless he means that we are no longer under that particular law, with which I agree. Loads of Christians are circumcised throughout the world for all sorts of reasons but in the case of Christians, it is not a religious requirement as it is for Jewish people (& for Muslims). Anything that smacked of slavery to 'the law' (circumcision not being the only one), was discouraged in the early days of Christianity but not forbidden and there is nothing inherently wrong with the practice. There are several Biblical references, I'll find them.
Paul speaking: Galatians: 2 1-5 Then after fourteen years, I went up again to Jerusalem, this time with Barnabas. I took Titus along also. 2 I went in response to a revelation and, meeting privately with those esteemed as leaders, I presented to them the gospel that I preach among the Gentiles. I wanted to be sure I was not running and had not been running my race in vain. 3 Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek. 4 This matter arose because some false believers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves. 5 We did not give in to them for a moment, so that the truth of the gospel might be preserved for you. NIV
Here is the reference to Timothy, mentioned earlier: Acts 16:3: Paul wanted Timothy to accompany him, and he took him and circumcised him because of the Jews who were in those places, for they all knew that his father was a Greek. ESV
There are numerous Biblical references to circumcision but not much point in me printing them all here because if anyone is interested they can google for themselves.
It's quite clear that circumcision is not necessary for Christians but not wrong in itself.
Even the covenant with Abraham was first and foremost a spiritual thing, the circumcision was an outward sign of the covenant so that he and his descendants, as God's chosen, were set apart from others.