I built the design that Atmel provides on their web site, it does work but I found that I had to remove the 10uf capacitor in the 12v/5v reg to make it work.

Their are other designs for programming the 2051 which require only a 5v reg and a parallel port connector. I can't provide a URL at the moment as I'm currently at work and don't have it with me, just use a search engine and look for 'at89c2051 programmer'.