[SalesForce] Is it possible to cast an sObject to another object that is dynamic

Here's my stab at what I think you're trying to do:

public class GlobalIds {

    public list<sObject> generateGlobal_Id_Generator(list<sObject> TheRecords){

        if(!TheRecords.isEmpty()){ 
            // I guess you mean to check that each element matches the sObjectType
            // reported by the list, so...
            for(sObject Record : TheRecords) {
                system.assertEquals(TheRecords.getSObjectType(),Record.getSObjectType());
            }
            return getGlobal_Ids(TheRecords, TheRecords[0].getSObjectType());

        }else{
            Throw new CustomIdGenerationListIsEmptyException();
        }
    }

    public list<sObject> getGlobal_Ids(list<sObject> TheRecords, Schema.SObjectType objectType){

        for(SObject Record:TheRecords){
            // Using .put() instead of assignment because it's a generic sObject
            // Using objectType.getDescribe().getName() because we've already checked that
            // the sObjectType is correct
            Record.put('Global_Id__c', getGlobal_Id(objectType.getDescribe().getName(), Id.ValueOf(Record.Id)));
        }   
        return TheRecords;
    }

    // Mocks so that I can compile this
    public String getGlobal_Id(String objName, Id objId) {
        return objName + objId;
    }
    public class CustomIdGenerationListIsEmptyException extends Exception {

    }    
}

I'm not sure that I can explain much more than just what's in the comments, but happy to answer questions on it. Or be told that I missed the point ;-)

So I made a Map fieldTypeMap and a component to handle the date/datetime format

Here is what I did, maybe it will be usefull for someone else.

Controller

I added the map as an attribute of my inner class into the controller:

// header/type mapping
public Map<String,String> headerTypeMap {get;set;}

I initialize it at the end of the constructor, thanks to this.headers (array of string header/field name), after the queries, like this:

        // calculation :
        runQueries();

        // mapping
        this.headerTypeMap = new Map<String,String>();
        for(String field : this.headers){
            this.headerTypeMap.put(
                field,
                Schema.getGlobalDescribe().get(objType).getDescribe().fields.getMap().get(field).getDescribe().getType()+''
            );
        }

Visualforce component

Because I often need to format date and datetime I thought it could be a good idea to create a formating component So I made a quick and dirty one:

<apex:component controller="FormatedValueController">
<apex:attribute name="dateValue" 
    type="Date" 
    assignTo="{!inputDateValue}"
    description="input value to formatTodoBlock__c record that contains data" 
/>
<apex:attribute name="datetimeValue" 
    type="Date" 
    assignTo="{!inputDateTimeValue}"
    description="input value to formatTodoBlock__c record that contains data" 
/>

{!formatedvalue}

Controller:

public class FormatedValueController {
    public Date     inputDateValue {get;set;}
    public Datetime inputDateTimeValue {get;set;}

    public String getFormatedValue(){
        String formatedValue = '';
        formatedValue += inputDateValue == null ? '' : inputDateValue.format();
        formatedValue += inputDateTimeValue == null ? '' : inputDateTimeValue.format();
        return formatedValue;
    }
}

And a quick&dirty test class too (for code coverage only):

@isTest
global class FormatedValueController_test{
    static testMethod void FormatedValueController() {
        FormatedValueController c = new FormatedValueController();
        Test.startTest ();
        c.getFormatedValue();
        Test.stopTest();
    }
}

100% you-hou ;)

And finally ...

Main Component

Oddly I need to use "{!todoBlock.headerTypeMap[header]}" everywhere, where todoBlock is the instance of my innerclass and header the var of repeat on headers attribute (table with a dynamic number of columns).

I wanted to use an apex variable for each header, kind of:

<apex:variable var="type" value="{!todoBlock.headerTypeMap[header]}" />

but for an unknown reason it just display the last header value in every loop. For example if I have DATE, DATETIME, STRING, it displays STRING, STRING, STRING. Not a big deal to use the full code, it is just less readable.

So here it is :

<apex:outputLabel rendered="{!todoBlock.headerTypeMap[header]=='DATETIME'}">
    <c:FormatedValue datetimeValue="{!rec[header]}"/>
</apex:outputLabel>

<apex:outputLabel rendered="{!todoBlock.headerTypeMap[header]=='DATE'}">
    <c:FormatedValue dateValue="{!rec[header]}"/>
</apex:outputLabel>

<apex:outputLabel rendered="{!NOT(AND(todoBlock.headerTypeMap[header]=='DATETIME',todoBlock.headerTypeMap[header]=='DATE'))}">
    {!rec[header]}
</apex:outputLabel>