should US citizen use US passport or HKID card in/out HK?

首先直接地回答你嘅問題 -- 你入香港境同離開香港境時, 應該使用香港身份證; 而你入番美國境之時, 就用美國護照, 呢個係正常及可接受做法.
你現持有美國護照, 即係美國公民; 你亦持有HKID, 我推斷你係指香港永久居民身份證 (Hong Kong Permanent Identity Card), 咁就代表你係香港居民, 代表你有香港居留權 -- 如果你翻轉你張智能身份證, 會睇到上面印住"本證持有人擁有香港居留權 The holder of this card has the right of abode in Hong Kong". 要清楚知道, HKID只係代表你有香港居留權, 同你咩國藉係冇關系. 再者, 香港並唔係一個國家, 何來"香港國藉"之有? HKID嘅情況就有如美國Green Card (全名為United States Permanent Resident Card) 一樣, 只係代表你可以有美國嘅居留權, 唔代表你有美國籍 -- 況且, 當你入美國籍後, 你就要交還greencard. 呢個就係公民同居民嘅分別.
例子: 前幾個月中國鋼琴家李雲迪獲香港身份證, 即係佢可長期定居香港而唔需要幾個月去更新佢入香港嘅簽證, 但佢仍然係中華人民共和國嘅公民, 攞中國護照.
喺美國入境過移民局時, 個官會問你去過邊度, 去咗幾耐, 原因係乜(旅遊或公幹) 但一般唔會問你用乜嘢證件進出當地. 就算問到, 你冷靜而有禮貌咁解釋你有香港居民身份證進出香港就得. 況且美國唔一定有你出境紀錄 -- 你答飛機離美國境時, 只係機場地勤人員核對你身份同旅遊證件. 你揸車去加拿大時, 都係直接去到加拿大入境, 而冇美國出境呢個手續. 就算美國要知道你喺外地用咩證件入境, 呢D係牽涉外交, 除非你有可疑, 如果唔係查你做乜呢?
至於雙重國籍, 美國係唔會理會(除非你嘅工作同國家安全有關, 亦視乎你有咩國家國籍), 所以你入番美國境可以放心. 雙重國籍呢個話題同呢條問題扯唔上邊, 所以唔仔細談論. 你可以到以下美國政府官方網頁查看.
http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html

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You MUST use US passport to leave the US, otherwise you'll have trouble reentering US. You can still use HKID for other countries but not US. US does not recognize dual citizenship. When you swore to be a US citizen, US recognize you only as a US citizen, but not a citizen of other countries. Therefore, by using HKID to leave US, they will doubt about if you still want to be a US citizen. However, even thought US does not recognize your HKID, other countries, including HK recognize it. So you can still use your HKID in countries other than US.

As a HK citizen it's your right to enter HK using you HK ID card.
It doesn't matter if you have citizenship in multiple countries, you can choose the way you like, to be treated as a foreigner or a local resident, when you enter a country. Me, myself as a british citizen, i enter/leave HK using my ID card, as it's much quicker, but enter other nations including UK with by UK passport and so far, encounters no problem at all. So i would recommand you to do the same way with your US passport which should save you some time passing thru the HK border.

hope it helps.^^

2008-02-21 14:37:18 補充：
bytheway, I've got a HKSAR passport and HKID and i am still keeping my uk citizenship, so multiple citizenship should applies in hk.