<p>We're at the beginning of a new ERP-ish client-server application, developed as a Python rich client.  We're currently evaluating Dabo as our main framework and it looks quite nice and easy to use, but I was wondering, has anyone used it for medium-to-big sized projects?<br>
Thanks for your time!</p>

I have no Dabo experience at all but this question is on the top of the list fo such a long time that I decided to give it a shot:
Framework selection
Assumptions:
medium-to-big project: we're talking about a team of more than 20 people working on something for about a year for the first phase. This is usually an expensive and very important effort for the client.
this project will have significant amount of users (around a hundred) so performance is essential
it's an ERP project so the application will work with large amounts of information
you have no prior Dabo experience in your team
Considerations:
I could not open Dabo project site right now. There seems to be some server problem. That alone would make me think twice about using it for a big project.
It's not a well-known framework.  Typing Dabo in Google returns almost no useful results, it does not have a Wikipedia page, all-in-all it's quite obscure. It means that when you will have problems with it (and you will have problems with it) you will have almost no place to go. Your question was unanswered for 8 days on SO, this alone would make me re-consider. If you base your project on an obscure technology you have no previous experience with - it's a huge risk.
You don't have people who know that framework in your team. It means that you have to learn it to get any results at all and to master it will require quite significant amount of time. You will have to factor that time into your project plan. Do you really need it?
What does this framework give you that you cannot do yourself? Quite a lot of time my team tried to use some third-party component or tool only to find that building a custom one would be faster than dealing with third-party problems and limitations. There are brilliant tools available to people nowadays and we would be lost without them - but you have to carefully consider if this tool  is one of them
Dabo project version is 0.84. Do you know if they spend time optimising their code for performance at this stage? Did you run any tests to see it will sustain the load you have in your NFRs.
Hope that helps :) Good luck with your project