I've noticed that when i'm home and wifi is on, it will work for a while then stop and switch to 3G. I know my router and wifi signal is fine and strong because my computer and xbox never ever problems, just my Hero.
I can shut the wifi off then on then it picks it up again or i have to go into the wifi settings, where under my network "linksys" it says "connection unsuccessful, select to try again". And i can re initiate the connection that way.
But i guess i'm asking is why it fails in the first place, and it happens almost daily after a while. My computer, laptop and xbox never fail so i know it's not the hotspot itself.