Technically, it's incorrect to use 'hopefully' at the start of a sentence
when you mean 'It is to be hoped'. The usage of the word as a sentence adverb
was probably derived from the German adverb
'hoffentlich'. 'Hopefully' should really only be used in sentences such as "We travelled hopefully towards the treasure." (ie. 'full of hope')
Although grammatical purists have strongly objected to the incorrect usage of 'hopefully' for many years, the new meaning of the word has become so rooted in people's vocabularies that changing it now may be impossible. After all, the new meaning is far more useful in everyday speech (the German language uses the distinct word 'hoffnungsvoll' to mean 'it is to be hoped').
Nevertheless, it's still wise to avoid the word in any critical situations (for the moment, anyway) - applying for a job as an English teacher, or even on any CV perhaps.
Ref: The Right Word at the Right Time (Reader's Digest Publication), pp.271-273