Hi,
Since I wrote phpPgAdmin, perhaps I can best answer that 🙂
CREATE UNIQUE INDEX form is a postgres-only extension for creating unique indexes. Before say PostgreSQL 7.2 or so, there was no way of creating a unique constraint. A unique constraint is simply a unique, btree, non-partial, non-expressional index.
Basically, the reason why the phppgadmin create index screen lets you create unique index is so you can create unique rtree indexes, unique partial indexes, etc.
There really is no difference between a unique btree index and a unique constraint - so don't stress too much about it.
Chris