Limitations of MySQL for this application may be why Twitter is spending a bunch of money looking at file system-based and other options for their nextgen architecture. - Ken Sheppardson

The identi.ca db architecture is such that, compiling a user's stream will result in a very heavy db load when the person follows many folks. Exactly the same issue that Twitter is trying to solve. - Dewald Pretorius

Perhaps, but I have high hopes that the brightest minds in the world can fix this. It is open source, after all. - Hao Chen

Agree, Hao Chen. Twitter's been at it two years full time with significant venture funding and still have problems, so it's apparently not something that we're likely to see solved overnight with off-the-shelf PHP library ORM libraries and basic MySQL tables. This could take a while. - Ken Sheppardson