Another update of SMS Popup is now in the Market. Thanks goes out to Diego Fernández for translating the app into Spanish - I'm not actually sure it's possible to switch any Android devices into Spanish locale yet (except the emulator), but once it is possible the translation is already included in the app :)
SMS Popup v0.9.7 & v0.9.8
Fixed crash (force close) when receiving MMS messages without a subject (hopefully)
Added an option to show a Delete button on the popup (by popular demand)
Update: v0.9.7 had a minor bug that prevented messages being deleted when using the new delete button - v0.9.8 fixes this (although I suspect it is still broken for deleting MMS messages).