PostgreSQL 9.1 vs MySQL 5.6 with InnoDB ? Which is better for scaling?
In scalability, part of the problem is that this is pretty heavily use-case dependent.  There is no scaling solution that hits every use case equally. For example, Slony is very helpful for scaling out in some cases, but is a bear in others.  For example, Slony lets you replicate only part of your database.  On the other hand, Slony lets you replicate only part of your database....  Overall, MySQL seems a little easier to scale out of the box in the workloads it supports but PostgreSQL seems to offer more advanced tools which you can take quite a bit further.

As far as which is more scalable in common use cases, now that Postgres-XC has been released, PostgreSQL is the clear winner here.   We have Slony, Bucardo, Streaming Replication, PGPool, etc. on the replication side.  We have Postgres-XC on the scale out clustering side.  We are extremely scalable now.

However, all these come with complexity costs.  These costs appear a bit higher on the Pg side, but the flexibility is there to scale however you need to.

Scaling up right now is also a major PostgreSQL goal.  9.2 makes some important gains in that regard.