Your comments

I played around a bit with, but it was quite slow to respond.

I tried to compile a piece of C# source code I had lying around, but it didn't have the Windows Forms assemblies as it uses the Mono compiler. So it was pretty useless for C#.

For things like PHP or Python it might look different though.

Currently, I'm not quite convinced that it would be a really useful feature.

I thought I can use it as a way to check the syntax of files without actually running the code.
Thanks for pointing that out!
Customer request by mail: "I would like a convert tabs to spaces option."
Comment received by mail: "I don't think that the information not copying over when moving the file would be all that big a deal unless you were moving control of the file over to a new location and deleting it from the device. In cases like that you might consider a safety feature that warns you that you'll lose that data and then have it offer to copy that information to a separate txt file to send along witih the original so that the information is not lost."
Yeah, that's certainly an interesting idea. That way it would also be included in the actual file (in comments) and available to other applications as well. Textastic could display a list of TODO items.

That reminds me of this idea:

The scratchpad, which is only available in Textastic, could be an additional/separate feature
I already store information like last cursor position and custom syntax definition setting for each file in the upcoming 1.1 update of Textastic. It should be relatively easy to add a scratchpad feature. I might add an auto-resizing textfield in the file info popover. People could use this for whatever they want to note about the file.

The only downside I see is this: it wouldn't be copied over when copying the file to another app or computer or if you send it via email.
Implementing option 1) would be a whole lot easier if the Dropbox API supported sync out of the box.

Is anyone aware of an existing sync implementation for the Dropbox iOS API with source code available?
I just had a look at Terra Web Browser. Seems like they use a mixture of 4.2 and the 4.1 SDK:

DTCompiler: "4.2"
DTPlatformVersion:"4.1 GM"
DTSDKName: "iphoneos4.1"
MinimumOSVersion: "3.2"

Well, I'll rather play it safe instead of having Apple reject the app.
Well, as long as this is in beta I can't submit iPad apps compiled with the 4.2 SDK but I have to use the 3.2 SDK.

But I tested version 1.1 with 4.2 beta 3 and it's working like a charm. Of course I'll be sure to submit an update as soon as Apple allows this.