The first one is the standard balance between giving away code and making money from it. That's often managed by withholding some features for a version put out under a commercial licence, while releasing a community edition to hoi polloi. That works, but isn't perfect, because it means that the latter feel that they're being exploited (via their indispensable feedback) from the versions they help to create, but can't have without coughing up for.
Funambol avoids this tension by making use of some deft market segmentation. Open source versions of its products are aimed at enterprises; the commercial version isn't simply a superior version of that, but an entirely different animal, aimed at a different market (carriers, ISPs, portals etc.). That way, Funambol can give full open source versions to general users, who provide the feedback, while selling specialists versions that wouldn't be of interest to enterprises anyway.
The second tightrope is one of acceptability. The idea of pushing ads along with email to mobiles is not new; the trick is the manner in which you do it. The Gmail approach, with ads as an integral part of the message, won't work because there is insufficient screen space available: they would just become intrusive. Funambol's solution is to use mini-banner ads along the top of the mobile screen. These are relatively unobtrusive, and let people get on with the business of reading their mail.
The final tightrope is more tricky, since it involves privacy. One of the most interesting applications of Funambol's technology is providing targeted ads that know who you are and – most intriguingly – where you are: think special offers in a shop you are just passing that very minute, or restaurants in your vicinity if it's near a mealtime. That kind of targeted marketing is great for advertisers, and potentially good for users. But it also implies that someone – probably not Funambol – knows a great deal about you, even down to where you are that very moment. Striking a balance between using and abusing that kind of personal info is going to be hard. If Funambol's technology or equivalent becomes widespread, we will all need to become funambulists.