Seeing as nobody had an answer to the 31-March-2008 expiry issue, Marriott Concierge Marie kindly looked into this question. Apparently it is an ongoing promotion and the references to a 31-March deadline (on both the web site and the Emails that referrals generate) are incorrect.

Note that when signing up through referrals or other promotions handled by joinmarriottrewards.com, you have to wait 48 hours before you can create a login password to view and manage your account online. The "Welcome" Email currently encourages you to create online access without mentioning the 48 hour delay. This 48 hour delay does not apply to regular signups made at marriott.com, because that registration page has a mandatory section called "Sign In Information" which enables online access at the same time that it creates your MR account. Phone representatives, however, will not be able to easily find your information until about 48 hours later, regardless of which site you use to join.

Many thanks to Marie for her patience and efforts getting to the bottom of this question.

Incidentally, it appears it may be possible to use both the referral and Cathay or BA promotions for maximum initial signup benefit.