I can't comment on what the SQL Team are thinking now. But can say the idea of an in-memory DB is not new & is something Microsoft has invested a lot of time exploring.

In the past they found that the benefits were marginal for most situations. In fact when you tried to cater for the transactional demands & the need to stay working when you run out of RAM, the design of the in-memory DB converged with the existing SQL server product. So they choose to take some of tht work to continue to optimise SQL so that it will perform well as an IMDB for those niche situations that would benefit from one.

If you think about how SQL's Buffercache works it, the tables (or pages) that you use often, remain in memory. If you do have more RAM than data, you will find SQL Server does behave as an IMDB with similar perf gains.

The issue, as you point out, is the need for log writes & persisting changes to disk. Even TempDB, while it may get Reset on Start, needs to log its changes if it is to be able to rollback any individual transaction to a savepoint.

If you have an application that is suited to the niche gains of an IMDB; (i) more RAM that data & (ii) essentially Read-Only. Consider setting the DB Option to Read-Only, this reduces the locking overhead, & frees RAM used by locks. Also using SQL 2008's Data Compression & table partition features may help. Putting your data on Read-Only Partitions may assist you reduce your maintenance window, while batch refreshing the non-Readonly bits of your database. It is also possible to spread your DB over 2 databases, where one is read-only. the usual issue of keeping the backups in sync is a non-issue as the RO db doesn't change. 🙂

Clearly H/W technologies like SSD can also give big gains. NB: Becareful here, SSD's are not "just RAM" many have a nasty drop in performance when they become near full. As a write few, read many device they are brilliant. Streaming large block writes they also handle well. But small random writes, can cause perf to degrade below a high speed disk. This perf drop varies from vendor to vendor & needs to be researched well.