Is there any problem for a boy and girl who are dating using nicknames for each other? i remember Rambam mentioning that this should be avoided although i have not found it. someone once told me that especially if one is not yet engaged using romantic nicknames should be avoided.

The period of dating and the ensuing period of being engaged pose many challenges as far as staying within the framework of halacha, modesty and holiness which are the ways of the Torah. Any infringement upon these values can lead down a slippery slope. Therefore our behavior during this time should be with caution. I believe the motto to be it adopted during this time are the words of Rabbi Akiva in Pirkei Avot (Chapter 3:13) "Sechok veKalut rosh margilin le' Ervah", meaning jesting and frivolity lead to lewdness. Therefore, apply this guideline to every situation in question and exercise restraint when it comes to words of affection. May your keeping of these values escort all the way to the "chuppa."