researching the topic for some time now, here are the two facts I've found so far from Microsoft, followed by a whole bunch of questions.
In Exchange 2007, you should use universal groups for distribution groups. Global and domain local groups can still be used when migrated, but are not recommended.
Because the members of universal groups are stored in the global catalog, you should be careful when using universal groups, especially when the membership changes often, because any change causes the whole list of group members to be replicated between the global catalog servers.
The third fact I've still not found is the first question:
1) What is the recommended best practice for distribution groups to avoid any problems? How should the members be organized?
The idea is to organize the final mail recipients into global groups (security, not mail-enabled) and to only put the global group into the universal group (distribution, mail-enabled). Some old groups were already organized this way, when Exchange 2007 exposed a bug that did prevent the mail distribution to the members of the global groups. A workaround was to mail-enable the global groups. AFAIK this bug is fixed.
2) Are the any other pros and cons for this practice (recipient -> global -> universal)?
To give some numbers, we are talking about a dozen domain controllers in different countries, each is a global catalog server and there are about 700 distribution groups with up to a few hundreds recipients.
3) What are the rule-of-thumb numbers when you should start worrying about it? Are there any informations available to estimate the replication traffic between the global catalogs based on the different actions?