I practice primarily in the Second and Third Circuits, because I am admitted to the courts in New York and New Jersey and have offices in these states.*
Here is the judicial standard for likelihood of confusion in the Second Circuit.
Here is the judicial standard for likelihood of confusion in the Third Circuit.
And here is the standard for likelihood of confusion in the Federal Circuit.
Someone else has already gone through the trouble of laying out the likelihood of confusions standards for all the Circuits in one place. Far be it from me to re-invent the wheel!
* I have, however, also appeared in litigation in the federal courts in Wisconsin, Arizona, California, Pennsylvania, Utah and Nevada. I am not admitted to practice in these states (I am admitted to the Eastern District of Wisconsin), but have been admitted pro hac vice for specific cases, a courtesy that is readily extended to foreign attorneys when certain requirements are met.