If you plan to go to Petrus on the 56/F of the Island Shangri-la, Jackets are required at night. A shirt and long pants will do the trick during lunch. It is a super fine restaurant with mignificant view of the harbor. I would highly recommend it, and lunch only starts at HK338.

Anywhere else I have been to, shorts are specifically not welcomed in hotel restaurants in Hong Kong. If you go to the Ritz-Carlton, you have to be sure to be specfically more formal as well. Mandarin is usually ok with dress shirts and long pants. T'ang Court in the Langham specifies no shorts as well.

However, those are to the top restaruants of the respective hotels. It is obvious that in a cafe envrionment, it will be far more relaxed.