Is the speed of a PostgreSQL SELECT adversely affected by too many indexes on the table?
No, the query performance will not be affected, or not very much.  Indexes are updated on DML statements (and TRUNCATE) while they may or may not be used when executing a query.

The decision whether they will be used are made by the planner.  With very many indexes present I can imagine that the planner spends more time on choosing the usable ones, but I would expect the difference to be small.  (Normally, planning is faster that retrieving rows.)

Note that usually a few indexes will speed up a lot of different queries.  This may not be the case if you filter on many of your columns.  

Also note that in a lot of cases (and without knowing your table definition one cannot decide whether your case falls into 'most' or not) having so many columns reflects having a less-that-optimal schema design.