A friend of mine managed to hit a chain of 40 and get a shiny, and he had the following observations:
- 4-space rule doesn't seem to apply; you just need to enter a bush that shakes identically to the one you first stepped into
- Edges
may be safe (interesting if so, and would explain why there aren't so many large patches of grass then - but assume not if not testing)
- Bushes that rustle a tiny amount will kill the chain if stepped on
Hope that helps you guys. I'll be trying the Pokeradar out myself, but I won't beat the game anytime soon. =/
Still not convinced by tipping. You won't hear stories of people saying 'I didn't tip so-and-so, and found a shiny!' after all. =p Plus there's millions of players, so there's going to be a bunch of shiny Pokemon found, particularly with the likes of hordes (5 chances at once!). If it's not mentioned in a guide, it'll be a very hard thing to verify too.
Tipping is in the game for a reason, so it'd be nice to know what the reason is, even if it's not boosting one's shiny encounter rate!
There was a donate hearts (currency of the game) in Kid Icarus: Uprising feature too. That did absolutely nothing bar have a character walk towards the screen. So I wouldn't be too quick to say that tipping is in for a (useful) reason. =p