It's a shame I haven't blogged for two months, but I've been really (I mean - REALLY) busy finishing the upcoming Microsoft Programming LINQ book. Now I'm coming back to my BI world and the first news is that I'll speak about SSAS models based on many-to-many relationships at the European PASS Conference 2008, April 14-16 2008, Neuss, Germany (Europe, of course...). For those of you who want to know what I'm going to talk about... you can read The Many-to-Many Revolution paper I wrote more than one year ago. It is based on SSAS 2005, but there is nothing to add with SSAS 2008 too.
In reality I and Alberto developed some variation of the base models I described in the paper. In particular, we had great success with the Balance Reclassification model (sorry, we still don't have an article about it) that allows defining multiple parent-child hierarchies on a balance sheet, also using sign rollup operator (which is not natively supported if you have a many-to-many relationship with a parent-child hierarchy). Unfortunately, we desisted using it in a Gb-sized fact table for performance reason. But it is working well in many other typical scenarios, where balance sheet fact table has a granularity that gives a lower size of the fact table.
Let me know if this is an argument (Balance Reclassification model) that is interesting for you - I could give some anticipation in a blog post or in a short article.