I just want to chime in and say that mine does that. Funny thing is that it only started once I started using custom ROMs. When it happened to me I was using Mighty ROM. The first time it happened I thought the keyboard was completely dead. I did about 3 soft resets with no luck and then I did a hard reset with no luck.
I then downloaded the stock Sprint ROM and the keyboard started working again, perfectly. I'd like to know what the hell is causing this.
Since then, I've put back the Mighty ROM since it's the ****, but even now my keyboard dies about once a week.
(I'm not blaming Mighty ROM, I'm just letting you guys know how it happened to me)
edit: After I wrote this post I took a shower for work and when I came out I tried to text my girl and sure enough the keyboard isn't working again. A soft reset just fixed it.