S3 and EC2

I have a small webapp that I am in midst of launching. I think it fulfills a common need effectively yet is dead simple to use (at least my wife and son didn't need any instructions). One problem the webapp has is that it needs to ping the server once a second or so. Storage wise, I am expecting 50K per session so it's not unusually high. Even with smart ping rate logic, server load will be much higher than your average webapp.
During a chat with a friend, S3 came up (I keep forgetting) so I quickly found a way to shove most of the server load to S3. But I overlooked the 1 cent per 10K request cost. Assuming average of 100K active sessions, S3 will get hit with 100K requests per second which will cost me well over $8000 per day just for requests. Since most of the unpaid-for-data won't survive more than 24 hours, storage cost is bearable and so is the transfer cost because most of those requests will return nothing. I know that 100K simultaneous users is rather high but that's the bill I will have to face if I get slashdotted, farked, or digged and I do expect the webapp to be popular enough to get that kind of load.
Thankfully, EC2 doesn't have request-related cost so I am going to look into that. I am concerned about reliability though since they are still in beta phase. At worst case, I am going to launch with a handful of dedicated or co-located servers which should cost me about $1000 per month then add as demand increases. If you have good suggestions, let me know. Come to think of it, my webapp's requirements are similar to applications like Hive7 so I'll be asking them as well.