Now of course, you can always go to the "Trash" folder and recover those deleted mails... but on a mobile phone.. that is a long process, you know, you gotta tap this and that button and then use your fingers in a weird ways (thanks to Apple :P).
But now, whenever you're using g-mail mobile web app and delete a message you'll see a "bar" at the bottom of your mobile g-mail screen in which you can access the "Undo" button (see the below screenshot).
Not just "undo" button, but this Yellow bar gives you four main functions according to the Google mobile team, which are...
*. Add/Remove a label.
*. Move message/conversation.
Currently this features are supported by Android OS, iOS and BlackBerry OS 6.0 phones only.
Anyway, the next time you use G-mail mobile web app make sure to get the maximum out of this cool feature.