I have been a fall of twitter for sometime now, but they are having serious problems due to their architecture .
(See here Twitter Technology Blog: Twittering About Architecture). I hope they get it resolved soon, before it is too late.
I have also been a fan of tumblr for some time too. So I thought it was interesting that twitter is updating their
Twitter Status using tumblr. Perhaps more people will start using tumblr as a result. Tumblr is great and worth considering.
Smart People I Know 
I wonder if it is because of the platform of choice. They used Ruby, but hopefully not RoR. For some thing as relatively “small” in terms of data RoR would’ve been a bit of an overkill.
I guess this is why I still prefer Java as most of the scalability issues in the frameworks are relatively taken care of. Especailly since Java itself is a resource hog at times, you’re forced to deal with it ASAP.
I’d probably put a C-JDBC cluster under a cluster of Tomcat/Jetty servers under a squid proxy cluster.
That way there is no single point of failure for a running application.
I’d probably put some DoS rules on either a resource filter or directly into Squid (if possible).
I used to map my Twitter feed into my tumble log as text posts. Now I just render the rss into the HTML/template with java. I love Tumblr and it’s far superior to twitter for ‘tweeting’, or updating your current status.
I just made a new Tumblr theme and its available for download @ SkyLite.Tumblr.com.
Come check it out and feel free to follow my tumble log @ chvnx.com