Thanks for the great tutorial. I made some changes when i implemented it to support a table that has more data than just the uploaded image, and i also figured out how to get the blob_info w/o creating a new dummy request. I also made it handle the deletion requests, and update requests (assuming that the same blob is only referenced in 1 record) my gae.py (there are different names for the tables and fields here, i was hoping to make it generic but have not yet done that)

from google.appengine.ext import blobstore
from google.appengine.ext import webapp
from google.appengine.ext.webapp import blobstore_handlers
from google.appengine.ext.webapp.util import run_wsgi_app
import uuid
import logging

#web2py controller, handle gae blobstore upload
def upload():
    logging.info("in upload")
    logging.info(request.args)
    logging.info(request.vars)
    #@TOD: make the table name and field parameters to pass in
    table_name = 'artwork'

    form = SQLFORM(db[table_name])
    if request.args and request.vars.id:
        form = SQLFORM(db[table_name], request.vars.id)

    if form.accepts(request.vars, session):
        logging.info("form accepted")
        row = db(db[table_name].id == form.vars.id).select().first()
        if request.vars.preview_image__delete == 'on' or \
            (form.vars.preview_image and row.blob_key):
            #remove from blobstore
            key = row.blob_key
            blobstore.delete(key)
            #remove reference here
            row.update_record(blob_key=None, preview_image=None)
        if form.vars.preview_image:
            logging.info("new image")
            #@TODO: delete old image if replacing.
            blob_info = blobstore.parse_blob_info(form.vars.preview_image)
            row.update_record(preview_image = \
                table_name+".preview_image."+str(uuid.uuid4()).replace('-','')+".jpg",
                blob_key = blob_info.key())
        crud.archive(form)
    else:
        logging.info("form not accepted")
        logging.info(form.errors)
        session.flash=BEAUTIFY(form.errors)
        #there was an error, let's delete the newly uploaded image
        if request.vars.preview_image != None:
            logging.info("deleting new image")
            blob_info = blobstore.parse_blob_info(request.vars.preview_image)
            blobstore.delete(blob_info.key())


    logging.info("all done")
    #Raise the HTTP exception so that the response content stays empty.  calling
    #redirect puts content in the body which fails the blob upload
    raise HTTP(303,
               Location=URL(r=request,c='form',f='artwork',args=request.vars.id or []))


def download():

    #handle non-gae download
    if not request.env.web2py_runtime_gae or not request.args[0]:
        return response.download(request,db)

    #handle gae download
    my_uploads=db(db.artwork.preview_image==request.args[0]).select()[0]
    if not my_uploads.blob_key:
        return None
    blob_info = blobstore.get(my_uploads.blob_key)

    response.headers['X-AppEngine-BlobKey'] = my_uploads.blob_key;
    response.headers['Content-Type'] = blob_info.content_type;
    response.headers['Content-Disposition'] = "attachment; filename=%s" % blob_info.filename
    return response.body.getvalue()