It is very important that your speaker and you understand the legal status of the recorded database. It is very wise that the speaker signs a statement before you start recording or at least talk to them ensuring they understand what you want to do with the data and what restrictions if any they require. Remember in recording their voice you are potentially allowing anyone (who gets access to the database) to fake that person's voice. The whole issue of building a synthetic voice from recordings is still actually an uninvestigated part of copyright but there are clear ways to ensure you wont be caught out by a law suit, or a disgruntled subject later.
Explain what you going to do with the database. Get the speaker to agree to the level use you may make of the recordings (and any use of them). This will roughly be:
free for any use
free to distribute to anyone but cannot be used for commercial purposes without further contract.
research use only (does this allow public demos?)
Also, don't lie to the speaker make it clear, what it means if their voice is to be released for free. If you release the voice on the net (as we do with our voices), anyone may use it. It could be used anywhere, from reading porn stories to emergency broadcast systems. Also note that effectively building a voice from a synthesizer means that the person will no longer be able to use voice id systems as a password protection (actually that depends on the type of voice id system). However also reassure them that these extremes are very unlikely and actually they will be contributing to world of speech science and people will use their voice because they like it.
We (KAL and AWB) have already given up the idea that our voices are in anyway ours and have recorded databases and made them public (even though AWB has a funny accent). When recording others we ensure they understand the consequences and get them to explicitly sign a license that gives us (and/or our institution) the rights to do anything they wish, but the intention is the voice will be released for free without restriction. From our point of view, having no restrictions is by far the easiest. We also give (non-exclusive) commercial rights to the voice to the speaker themselves. This actually costs us nothing, and given most of our recorded voices are for free the speaker could re-release the free version and use it commercially (as can anyone else) but its nice that the original license allows the speaker direct commercial rights (none that I know of have actually done anything with those rights).
There may be other factors though. Someone else may be paying for the database so they need to be accommodated on any such license. Also a database may already be recorded under some license and you wish to use it to build a synthetic voice, make sure you have the rights to do this. Its amazing how mainly people record speech databases and don't take into account the fact that someone else may build a general TTS systems from their voice. Its better that you check that have to deal with problems later.
An example of the license we use at CMU is given in the festvox distribution festvox/src/vox_files/speaker.licence.
Also note that there are legal aspects to other parts of a synthetic voice the builder must also ensure they have rights to. Lexicons may have various restrictions. The Oxford Advanced Learners' Dictionary that we currently use for UK English voices is free for non-commercial use only, thus effectively imposing the same restriction on the complete voice even though the prosodic models and diphone databases are free. Also be careful you check the rights when building models from existing data. Some databases are free for research only and even data derived from them (e.g. duration models) may not be further distributed. Check at the start, question all pieces of the system to make sure you know who owns what and what restrictions they impose. This process is worth doing at the start of a project so things are always clear.