How does Squiggle relate to JDBC?

Squiggle's only purpose is for performing text-manipulation of SQL. It does not use any of the JDBC classes. Typically you would use Squiggle to generate your statement, then pass it to JDBC for execution.

How does Squiggle compare to JDO, Hibernate, CMP, etc?

It doesn't. Squiggle is not a persistence layer. In most cases I would recommend using a persistence layer such as Hibernate or JDO, but there are some occassions where you just need to revert to a plain old SQL query.

Can I modify the Squiggle source code and use it in my own application.

Sure! Squiggle is open sourced under a BSD license. Please read the license thoroughly.

Does Squiggle support joins and subselects?

Yep.

Does Squiggle support INSERT, UPDATE or DELETE statements?

Nope. Squiggle's purpose is for building SELECT statements only. However you can extend Squiggle to support custom needs if you want.