Category for TouchXML to convert a node to JSON. It won't provide fully valid JSON from any XML data, but it met my needs.

CXMLNode+JSON.h

//
//  CXMLNode+JSON.h
//
//  Created by Jason L Perry on 1/24/10.
//  Copyright 2010 Ambethia. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "TouchXML.h"

@interface CXMLNode (JSON)
-(NSString *) toJSON;
-(NSString *) _toJSONWithAnonymity:(Boolean)anonymous;
@end

CXMLNode+JSON.m

//
//  CXMLNode+JSON.m
//
//  Created by Jason L Perry on 1/24/10.
//  Copyright 2010 Ambethia. All rights reserved.
//

#import "CXMLNode+JSON.h"

@implementation CXMLNode (JSON)

-(NSString *) toJSON {
  return [self _toJSONWithAnonymity:YES];
}

-(NSString *) _toJSONWithAnonymity:(Boolean)anonymous {
  NSMutableString *json = [[NSMutableString alloc] init];
  if (!anonymous) {
    [json appendFormat:@"%@: ", [self name]];
  }
  if ([[[self stringValue] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] > 0) {
    [json appendFormat:@"\"%@\"", [self stringValue]];
  } else {
    NSMutableDictionary *childElements = [[NSMutableDictionary alloc] init];
    for (CXMLNode *node in [self children]) {
      if ([node kind] == CXMLElementKind) {
        if ([childElements objectForKey:[node name]] == nil) {
          NSMutableArray *nodes = [[NSMutableArray alloc] initWithObjects:node, nil];
          [childElements setObject:nodes forKey:[node name]];
          [nodes release];
        } else {
          [(NSMutableArray *)[childElements objectForKey:[node name]] addObject:node];
        }
      }
    }
    if ([childElements count] > 0) {
      [json appendString:@"{ "];
      NSMutableArray *childJson = [NSMutableArray arrayWithCapacity:[childElements count]];
      for (id key in childElements) {
        NSArray *elements = [childElements objectForKey:key];
        if ([elements count] > 1) {
          [json appendFormat:@"%@: ", key];
          [json appendString:@"["];
          NSMutableArray *arrayJson = [NSMutableArray arrayWithCapacity:[elements count]];
          for (CXMLNode *node in elements) {
            [arrayJson addObject:[node _toJSONWithAnonymity:YES]];
          }
          [json appendString:[arrayJson componentsJoinedByString:@", "]];
          [json appendString:@"]"];
        } else {
          [childJson addObject:[[elements lastObject] _toJSONWithAnonymity:NO]];
        }
      }
      [json appendString:[childJson componentsJoinedByString:@", "]];
      [json appendString:@" }"];
    } else {
      [json appendString:@"null"];
    }
    [childElements release];
  }
  return [json autorelease];
}

@end

CXMLNodeJSONTests.m

//
//  CXMLNodeJSONTests.m
//
//  Created by Jason L Perry on 1/24/10.
//  Copyright 2010 Ambethia. All rights reserved.
//
//  http://www.xml.com/pub/a/2006/05/31/converting-between-xml-and-json.html
//

#import <SenTestingKit/SenTestingKit.h>
#import "CXMLNode+JSON.h"

@interface CXMLNodeJSONTests : SenTestCase {
}
- (NSString *) getJSON:(NSString *)json fromXML:(NSString *)xml;
@end

@implementation CXMLNodeJSONTests

- (void) testPatternTwo {
  NSString *xml  = @"<e>text</e>";
  NSString *json = @"\"text\"";
  STAssertEqualObjects([self getJSON:json fromXML:xml], json, @"Pattern 2 Failure");
}

- (void) testPatternFive {
  NSString *xml  = @"<e> <a>text</a> <b>text</b> </e>";
  NSString *json = @"{ a: \"text\", b: \"text\" }";
  STAssertEqualObjects([self getJSON:json fromXML:xml], json, @"Pattern 5 Failure");  
}

- (void) testPatternSix {
  NSString *xml  = @"<e> <a>text</a> <a>text</a> </e>";
  NSString *json = @"{ a: [\"text\", \"text\"] }";
  STAssertEqualObjects([self getJSON:json fromXML:xml], json, @"Pattern 6 Failure");
}

- (void) testPatternOneKinda {
  NSString *xml  = @"<e> <a></a> </e>";
  NSString *json = @"{ a: null }";
  STAssertEqualObjects([self getJSON:json fromXML:xml], json, @"Pattern 1 Failure");  
}

- (NSString *) getJSON:(NSString *)json fromXML:(NSString *)xml {
  CXMLDocument *parser = [[CXMLDocument alloc] initWithXMLString:xml options:0 error:nil];
  CXMLElement *e = [[parser nodesForXPath:@"//e" error:nil] lastObject];
  NSString *res  = [e toJSON];
  [parser release];
  return res;
}

@end