What data type should my widgets accept/return?
This approach is quite generic and serializing to and from strings should always work fine. You could also save the state of a widget to a file or send it over a network for recreating another widget from it.
Some potential issues or aspects to consider:

Localization: how to interpret the string regarding the culture, which format is the canonical format for comparisons.
Performance: some transformations might be time consuming, but I assume for human interaction that will be far fast enough.
I'm building a form class in python for producing and validating HTML forms. Each field has an associated widget which defines how the field is rendered.
When the widget is created, it is passed in a (default) value so that it knows what to display the first time it is rendered. After the form is submitted, the widget is asked for a value. I delegate this to the widget rather than just nabbing it from the POST data because a widget may consist of several HTML inputs (think of a month/day/year selector). Only the widget knows how to mash these together into one value.
Problem is, I don't know if I should have the widget always accept a string, and always return a string for consistency, or accept and return a data type consistent with its purpose (i.e., a date selector should probably return a DateTime object).
The philosophy behind my form class is "mix and match". You choose what widget you want, and what validators/formatters/converters you want to run on it. Which I guess lends itself towards "use strings" and let the developer decide on the data type afterwords, but... I can't think of a good but. Do you anticipate any problems with this approach?