What is the test for dismissing a habeas petition containing unexhausted claims?

The exhaustion rule set forth in 28 U.S.C. 2254(b) requires district courts to dismiss habeas petitions containing unexhausted claims. Rose v. Lundy, 455 U.S. 509, 510, 102 S.Ct. 1198, 71 L.Ed.2d 379 (1982). In the case of a "mixed" petition, containing both exhausted and unexhausted claims, a petitioner is given "the choice of returning to state court to exhaust his claims or of amending or resubmitting the habeas petition to present only exhausted claims to the district court." Id.