http://iphonedevelopment.blogspot.com/2009/05/device-detection.html
Update:
The author of the device detection code has been identified! Max Horváth dropped me a note letting me know that this lovely little bit of code was written by him. He's got a great blog that often covers iPhone SDK material also.
Somebody asked me over Twitter for how to detect what device the code was running on. Since a tree fell in my backyard the other day, right on the telephone lines and taking out my internet service in the process, I was working on an iPhone, and trying to answer a question quickly.

In short, I gave a wrong answer. My apologies to anybody who relied on it.

Here's some code, written by Max Horváth, that detects the device model correctly.

#import <sys/utsname.h>

enum {
MODEL_IPHONE_SIMULATOR,
MODEL_IPOD_TOUCH,
MODEL_IPHONE,
MODEL_IPHONE_3G
}
;

@interface DeviceDetection : NSObject

+ (uint) detectDevice;
+ (NSString *) returnDeviceName:(BOOL)ignoreSimulator;

@end



@implementation DeviceDetection

+ (uint) detectDevice {
NSString *model= [[UIDevice currentDevice] model];

// Some iPod Touch return "iPod Touch", others just "iPod"

NSString *iPodTouch = @"iPod Touch";
NSString *iPodTouchLowerCase = @"iPod touch";
NSString *iPodTouchShort = @"iPod";

NSString *iPhoneSimulator = @"iPhone Simulator";

uint detected;

if ([model compare:iPhoneSimulator] == NSOrderedSame) {
// iPhone simulator
detected = MODEL_IPHONE_SIMULATOR;
}
else if ([model compare:iPodTouch] == NSOrderedSame) {
// iPod Touch
detected = MODEL_IPOD_TOUCH;
}
else if ([model compare:iPodTouchLowerCase] == NSOrderedSame) {
// iPod Touch
detected = MODEL_IPOD_TOUCH;
}
else if ([model compare:iPodTouchShort] == NSOrderedSame) {
// iPod Touch
detected = MODEL_IPOD_TOUCH;
}
else {
// Could be an iPhone V1 or iPhone 3G (model should be "iPhone")
struct utsname u;

// u.machine could be "i386" for the simulator, "iPod1,1" on iPod Touch, "iPhone1,1" on iPhone V1 & "iPhone1,2" on iPhone3G

uname(&u);

if (!strcmp(u.machine, "iPhone1,1")) {
detected = MODEL_IPHONE;
}
else {
detected = MODEL_IPHONE_3G;
}

}

return detected;
}



+ (NSString *) returnDeviceName:(BOOL)ignoreSimulator {
NSString *returnValue = @"Unknown";

switch ([DeviceDetection detectDevice]) {
case MODEL_IPHONE_SIMULATOR:
if (ignoreSimulator) {
returnValue = @"iPhone 3G";
}
else {
returnValue = @"iPhone Simulator";
}

break;
case MODEL_IPOD_TOUCH:
returnValue = @"iPod Touch";
break;
case MODEL_IPHONE:
returnValue = @"iPhone";
break;
case MODEL_IPHONE_3G:
returnValue = @"iPhone 3G";
break;
default:
break;
}


return returnValue;
}


@end