I thought Monk already did a pretty good job of extending Sinatra without going overboard.
I'd be interested to know how Padrino compares to Monk.
At first glance Padrino seems closer to Rails in scale - which seems to defeat the whole minimalist mindset behind Sinatra. I mean, if I want Rails, I'll use Rails.
The extensive community support for Rails in combination with the improvements in Rails 3 already makes it difficult to justify using a smaller framework like Sinatra.
Many of the questions I have asked in the Sinatra community have gone unanswered so I imagine using Padrino (or even Monk) would simply add to that problem.
On the other hand, with Rails, the combination of StackOverflow, Google Groups and a plethora of Googleble web resources means you are never far from resolving a holdup.