Yeh, I would agree with Dan.
I ran a 10 week course as an 'intro' to aikido. Started off with large numbers and rapidly dropped off. The problem was that most people have the 'death touch' mentality to martial arts at first and think that they will be expert in a few weeks.
Also, many people (esp. older people) are very frightened of doing ukemis and feel stupid doing them (how will a ukemi help me beat someone up?).
Ikkyo, Nikkyo (beginners always like Nikkyo 'cos they think it works, and they have doubts about everything else!) and shiho-nage are good starters., though they don't necessarily teach the core of aikido.
Don't do too much. Only 1 technique in a night. Also, make sure you include a few striking attacks during the course or they will think its only useable against grabs!