In this particular case, by OPs own admission his passport is expiring in May '09 and he plans to file the Tier 1 application in August '08 - that means passport is valid for approx 8-9 months at the time of application - so the 6 month passport validity condition (if there is one) doesn't come into play.
AFAIK the requirement that the passport must be valid for at least 6 months is usually applicable when applying for the standard short-term visas e.g. - 6 month visitor/business visa (to ensure that the passport doesn't expire while the traveller is in a foreign country, 'coz being a visitor he will not be able to renew his passport in that country under normal circumstances). This is my personal understanding.
In context of Tier 1, I have not been able to find any instruction / condition from UKBA that the applicant's passport must be valid for at least 6 months at the time of initial application or extension. Or is it that I haven't looked properly?
regards