Localhost is not refreshing/reseting
Those warnings shouldn't prevent you from seeing new 'content,' they simply mean that you are missing some libraries necessary to run local versions of CloudSQL (MySQL) and the Images API.
First to do is try to clear your browser cache. What changes did you make to your Hello World app?
I am absolute beginner using google app engine with python 2.7. I was successful with creating helloworld app, but then any changes I do to the original app doesn't show in localhost:8080. Is there a way to reset/refresh the localhost. I tried to create new projects/directories with different content but my localhost constantly shows the old "Hello world!" I get the following in the log window:

WARNING  2012-05-13 20:54:25,536 rdbms_mysqldb.py:74] The rdbms API is not available because the MySQLdb library could not be loaded.
  WARNING  2012-05-13 20:54:26,496 datastore_file_stub.py:518] Could not read datastore data from c:\users\tomek\appdata\local\temp\dev_appserver.datastore
  WARNING  2012-05-13 20:54:26,555 dev_appserver.py:3401] Could not initialize images API; you are likely missing the Python "PIL" module. ImportError: No module named _imaging

Please help...