I'll second what Robert suggested - be sure to check that their is not a conflict on interest with your existing job. I was surprised when I moved from the Private Sector to the Public Sector last year just what is considered a conflict of interest, as some of the "conflicts" are not obvious and many of the resulting constraints are very limiting. In the private world, what is considered a conflict is usually things like working for a competitor. In the public world, it is much more broad and limiting.
Furthermore, many municipalities use a more strict standard - essentially "potential perceived conflict of interest", which can be interpreted to mean one cannot do any side work in any way related to ones field. So depending upon your current organizations standards, you may be entirely precluded from doing any side work in the GIS field.
Be sure to also check if your organization requires disclosure paperwork. It may be for them that doing digitizing is OK, but you will have to get authorization first and then do regular disclosures.
As a practical example of this, when I won some prizes from ESRI a few months ago as part of the GeoNet Launch contest, I ended up turning down some of them as the City I work for would have considered it a potential conflict as the total value would have exceeded the limit my organization has set.
Chris Donohue, GISP