The approach you suggested is the first approach that i tried. And frankly it worked great when i am testing the feature on my dev macbook with just a couple of 1000's of users. But when i started testing it on our server loading entire legacy data redis ended up taking more than 6GB of RAM for abt half a million users.
i told myself to just plan for 2 times our current traffic and then later on think of a better solution(best thing wud be to plan for 5-10 times your current traffic) and told my CEO that we might have to bump up RAM on the redis machine to 32 or 64 GB in the future(the easiest way but not an elegant way to solve the problem is throw more resources). Also mind that you need atleast 2 such machines to provide failover. i just hoped salvatore(a great guy BTW) would release a cluster solution for redis and will save me.
While testing the feed we came to know we needed to add more such actions from the user and every time you add a new type of action for every user you are looking at increasing the memory of Redis process by substancial amount. you will run out of memory faster than you have planned. i thought VM is the solution to all of this as user feeds typically use only latest data and the entire feed doesn't need to be in the memory. But quite frankly VM has it's own set of problems -- http://groups.google.com/group/redis-db/browse_thread/thread....
Not telling that your solution isn't good. Just wanted to warn you about some potential problems from experience of trying out your solution. Finally i am using cassandra for news feed. Working great as of now. At least much better than my experience with implementing the same with redis. The major problem with cassandra as of now is implementing pagination and counting. distributed counters are coming in 0.8 . pagination is still a major problem though. But for news feed you might not need pagination. providing a load more button as facebook and twitter does should be good enough.
Hope it helps.