Using GUID Fields As Primary Keys W/ Foreign Key Constraints

Ok, so I've broken down and decided to write myself an invoicing
program.  I'd like to use GUID fields as the Primary Keys in my
table.  Now, I basicly bill for two seperate items:

Deliverables and Services.

So, my Layout's gonna look something like

Create Table Invoice(
     ID UniqueIdentifier Primary Key,
     -- Other Data
);

Create Deliverable(
    ID uniqueidentifier Primary Key,
    ParentInvoice uniqueidentifier,
   -- Other data);
--...

Im sure there are probems with that as it's written feel free to edify me as to what they are. 

my questions are such:

1) Does a uniqueidentifier field automagically get a value?
2) If not how do I generate one in TSQL?
3) If so, what do I use to store my Foreign Keys.
4) How do I declare my Foreign key constraints?