forcing them will most likely get them to cling to there designs even more. just explain them the reason why you think certain things should be done certain ways

e.g like epage mentioned those "bubbles" for each tweet, you can easily explain why this little addition to the UI is improving the whole app.

1. better readability
2. better spacing, so you will instantly be able to recognise which text belongs to which tweet and which user etc.

so if you just explain it to them, I am 100% sure they'll think about it and try their best to integrate the improvements you have to offer.

Already done, but I'm going to start a brainstorm, maybe we need to build a resource of stock images and items for developers, and a place where they can request additional elements...

Design section would be great for requesting additional elements and wiki page would work for your stock images.

But how do we strongly encourage developers?

Two thoughts on this one. First is that since most of these are open sourced projects you could fork and enhance the UI. Second is somewhat related in that you can "vote with your dollars" where in this case your dollars are the programs you install.

Combine the two and you'll find that in the long run survival of the fittest will eventually win out.

Something to consider: pure coders tend to be more receptive to guidance in their craft than interface designers. No decent coder wants his/her app running with bugs. They make it look bad.

Artists tend to think that their work is the best approach, so they're not as receptive to constructive criticism. They don't think in terms of "bugs". How can their art be flawed?

A coder also performing as artist/designer without the proper experience/skillset there is inclined to first take a utilitarian approach and fail when trying to incorporate clever graphic elements. They will have to be sold on the idea of someone else coming in to revamp that part of the user experience.

The best sales approach IMO is to first convince the developer he/she can better maintain the code logic by divorcing it from the UI. The next step is to propose yourself as the person to enhance and maintain the UI as a team member.

Not every coder will be receptive, but those with the users in mind will. We have examples in Dialcentral, eCoach, and others.

Developers who refuse to consider a "slick" UI will watch as competitors bury them in downloads.

EDIT: like Jsuggs said above.

disclaimer: I am a coder and artist.

That's where brainstorming comes in.

As pointed out by others earlier in the thread, Maemo, being a finger-oriented mobile OS, has some very specific UI guidelines for developers to follow to help maintain "consistent experiences". In my mind it really boils down to what you're doing though: Are you building an app to add/extend functionality of the device (i.e. a simple note taking app, a map application, etc), or are you building an app/program to be used/run on the device (i.e. a game, a photo editor, whatever)?

If the former, then you should definitely follow the visual requirements of the UI guidelines and do your very best to leave the "appearance" up to the hildon theme/ OS control in general. That way when a user changes themes, your intergrated app changes accordingly.

On the other hand, if you're building an "application" that conceptually stands out as a program rather than just some "feature compliment/way of doing cool stuff that you don't expect", then the interface should be more under your control than that of the underlying OS/theme. That's not to say you should ignore the UI guidelines about button placements, size of text/etc... those are all there to help ensure a pleasant experience on a mobile device... but in this type of situation things like PNG backgrounds, different looking buttons/etc are acceptable and almost expected, AS LONG as they're done well and the interface is INTUITIVE. Any time you deviate from the "Standard view" that users are used to, you have to put extra effort into making sure that they won't need an instruction manual in order to figure out how to use your design/UI elements.

Please don't do that. To your avarage linux user, nothing is more annoying then an app which doesn't adhere to standards. If you want a different look, you use another GTK theme - job done. I'm not 100% sure how it's on maemo, but I strongly suspect that there will be other themes. Please do not encourage the use of custom visuals, custom layouts or anything custom. It should be the choice of the user if he or she wants to use another style. Apart from that, both GTK and Qt are pretty flexible when it comes to different layouts.
Thanks.

Non-standard UI elements == Unthemable apps

If you're using custom graphics exclusively (e.g. Canola) that's one thing, but if you mix standard GTK widgets with custom backgrounds/buttons, it will be ugly (or unusable) in other themes.