Let me preface this by saying that I am very new to Apex, so there may be a better way to do this.  If anyone knows of a better way or a "best practice" please let me know.

The reason that I am putting the values in a map is because I am trying to make a generic web callout class.  My idea is to keep the name/value pairs close to the data and then let that call out to a central callout class.  In the example code above it is all together to give some context... but in actuallity there are two parts and they are separated.

This is what is really looks like:

trigger OpportunityCallout on Opportunity (after update) {
	for (Opportunity opp : Trigger.new) {
		String url = 'http://external.server.com';
		Map<String, String> dataPayload = new Map<String, String>();
		dataPayload.put('data[Event][name]', opp.Name);
		dataPayload.put('data[Event][start_date]', String.valueOf(opp.Arrival_Date__c));
		dataPayload.put('data[Event][end_date]', String.valueOf(opp.Departure_Date__c));
		dataPayload.put('data[Event][nightly_rate]', String.valueOf(opp.Nightly_Rate__c));
		dataPayload.put('data[Event][minimum_attendance]', String.valueOf(opp.Minimum_Attendance__c)); 
		dataPayload.put('data[Event][minimum_fee]', String.valueOf(opp.Minimum_Fee__c));
		dataPayload('data[Event][minimum_camper_days]', String.valueOf(opp.Minimum_Camper_Days__c));
		dataPayload.put('data[Event][deposit_amount]', String.valueOf(opp.Deposit_Amount__c));		
		
		// make the asynchronous web service callout
		WebServiceCallout.webPOST(url, dataPayload);

	}
}

So now the callout is pretty dumb and can be reused.  The one thing I am still working on is to figure out how to inteligently handle the response that is releavant to the calling trigger.

public class WebServiceCallout {
 
	@future (callout=true)
	public static void webPOST(String url, Map<String, String> dataPayload) {

 		String body = '_method=POST';
 		for (String Key:dataPayload.keySet()) {
 			String value = '';
 			if (dataPayload.get(Key) != null) { value = dataPayload.get(Key); }
 			body += '&'+EncodingUtil.urlEncode(Key, 'UTF-8')+'='+EncodingUtil.urlEncode(value, 'UTF-8');
    	}
 		
 		HttpRequest req = new HttpRequest();
 		req.setEndpoint(url);
 		req.setMethod('POST');
		req.setBody(body);

		HttpResponse res = new HttpResponse();
		Http http = new Http();
		res = http.send(req);
		//TODO: Do something with the response.
 	}
}