Enforcing Unique Constraint in GAE

I am trying out Google App Engine Java, however the absence of a unique constraint is making things difficult. I have been through this post and this blog suggests a method to implement something similar. My background is in MySQL.Moving to datastore without a unique constraint makes me jittery because I never had to worry about duplicate values before and checking each value before inserting a new value still has room for error.

"No, you still cannot specify unique during schema creation."

-- David Underhill talks about GAE and the unique constraint (post link)

What are you guys using to implement something similar to a unique or primary key?

I heard about a abstract datastore layer created using the low level api which worked like a regular RDB, which however was not free(however I do not remember the name of the software)

Schematic view of my problem

sNo = biggest serial_number in the db
sNo++
Insert new entry with sNo as serial_number value //checkpoint
User adds data pertaining to current serial_number 
Update entry with data where serial_number is sNo 

However at line number 3(checkpoint), I feel two users might add the same sNo. And that is what is preventing me from working with appengine.