I understand that, if you are married to a Thai citizen, a Non-Immigrant mutiple entry visa can easily be obtained in Penang. Would that situation also apply to Singapore?

In October 2001, I obtained a Non-O multiple entry one year visa in Singapore. I showed my original Thai marriage certificate and left them with a copy. The visa was ready the next day for a fee of S$ 92.00.

This type of visa still requires you to leave the country every 90 days.

Just to confirm that Jayenram was 100% correct. I did the run on 18 Mar 2003, no problems.