根据 Cambridge Advanced Learner's Dictionary （英国剑桥高级学习者字典）：
“Consider it done”的意思是：It is used to say that you will do a particular task immediately. 中文就是说，你会马上去把那件事做好了。个人觉得，这句话不只是含有“马上去做”的意思，而且含有“请放心，一定会完成”的意思。
例句：
"Do you think you could get this parcel mailed for me, please?" "Consider it done."
参考链接：http://dictionary.cambridge.org/define.asp?dict=CALD&key=16369