Can you publish the code so that we can get a clear picture?

private bool UpdateAccount(string strCorpCode, string strPartnerPlan, string strName, string strJoinDate, string strAddress1, string strAddress2, string strCity, string strProvince, string strCountry, string strPostalCode, string strAreaCode, string strPhone, string strExtension, string strHowTheyFoundUs, string strBillingInformationSupplied)
{
    double dCorpCode;
    if (double.TryParse(strCorpCode, out dCorpCode) == false)
        return false;

    DateTime dJoinDate;
    if (DateTime.TryParse(strJoinDate, out dJoinDate) == false)
        return false;

    bool bBillingInformationSupplied;
    if (bool.TryParse(strBillingInformationSupplied, out bBillingInformationSupplied) == false)
        return false;

    try
    {
        Account[] accounts                      = new Account[1];
        accounts[0]                             = new Account();
        accounts[0].CorpCode__c                 = dCorpCode.ToString();
        accounts[0].Partner_Plan_rdp__c         = strPartnerPlan;
        accounts[0].Name                        = strName;
        if (String.IsNullOrEmpty(strAddress2) == true)
            accounts[0].BillingStreet = strAddress1;
        else
            accounts[0].BillingStreet = strAddress1 + "\n" + strAddress2;
        accounts[0].BillingCity                 = strCity;
        accounts[0].BillingState                = strProvince;
        accounts[0].BillingCountry              = strCountry;
        accounts[0].BillingPostalCode           = strPostalCode;
        accounts[0].Phone                       = String.Format("{0}{1}{2}", String.IsNullOrEmpty(strAreaCode) ? "" : "(" + strAreaCode + ") ", strPhone, String.IsNullOrEmpty(strExtension) ? "" : " " + strExtension);
        accounts[0].Join_Date_rdp__c            = dJoinDate;
        accounts[0].Billing_Info_Supplied_rdp__c = bBillingInformationSupplied;
        accounts[0].How_They_Found_Us_rdp__c    = strHowTheyFoundUs;

        UpsertResult upsertResult = _sforceService.upsert("CorpCode__c", accounts)[0];
        if (upsertResult.success == false)
        {
            // TODO: Log error
            Error[] errors = upsertResult.errors;
            return false;
        }

        return true;
    }
    catch (Exception e)
    {
        // TODO: Log error
        return false;
    }
}

CorpCode__c is a string, Partner_Plan_rdp__c is a picklist string, How_They_Found_Us_rdp__c is a picklist string.  All of these update okay.

However Join_Date_rdp__c is a date and Billing_Info_Supplied_rdp__c is a Checkbox.  These do not update.

Thanks, Rob

See the sticky in the .NET forum titled "why is my date/number/boolean being ignored"