It is not necessary to use PEGylation in order to formulate an effective liposome based drug.
As a matter of fact, out of 15 promising liposome based drugs that are commercialized or are in phase III only 4 of them are PEGylated. These four drugs are: Doxil, Lipodox, Thermodox and Lipoplatin. See here:
Doxil and Lipodox (both PEGylated form of liposomal doxorubicin) have significantly more side effects than Myocet (the non PEGylated form of liposomal doxorubicin) and this is mainly due to the long circulation properties of PEGylated liposomes.
See the original Q&A here: http://www.quora.com/Is-it-necessary-to-use-PEGylation-in-order-to-formulate-and-commercialize-a-liposome-based-drug