My version of iMovie exports (09), by default, .m4v. (emphases mine)
The M4V file format is a video file format developed by Apple and is very close to MP4 format. The differences are the optional Apple's DRM copyright protection, and the treatment of AC3 (Dolby Digital) audio which is not standardized for MP4 container.
Google's Picasa service doesn't support .m4v ...
Video Upload Requirements : Video - Picasa Help:
... Uploadable Video File Types .3gp, .avi, .asf, .mov, .wmv, .mpg, .mp4, .m2t, .mmv, .m2ts ...
Neither does YouTube ...
WebM files - Vp8 video codec and Vorbis Audio codecs
.MPEG4, 3GPP and MOV files - Typically supporting h264, mpeg4 video codecs, and AAC audio codec
.AVI - Many cameras output this format - typically the video codec is MJPEG and audio is PCM
.MPEGPS - Typically supporting MPEG2 video codec and MP2 audio
.FLV - Adobe-FLV1 video codec, MP3 audio
Heavens, but I do hate video data standard issues.
It's the patents, it's the DRM, and above all, it's Apple. Data formats and DRM are at the core of Apple's great flaw -- a deep addiction to data lock .
 Pro video customers of Apple's Final Cut Pro are learning all about what Apple's data lock means.
Update 7/5/11: Two wikipedia articles on Apple's ProRes 422 and ProRes 4444 and these additional articles help capture the full horror of the 2011 state of video codecs -- and the complexity of the video editing workflow. There is nothing analogous to JPEG or even JPEG 2000. See also ...
- Apple ProRes - MultimediaWiki
- Final Cut Pro 7 - Working with Apple ProRes Codecs - "Apple ProRes is an extremely flexible post-production format. The post-production possibilities with Apple ProRes range from offline editing on a MacBook Pro to advanced “digital negative” workflows for theatrical releases on film."
- Final Cut Pro 7 - Types of Apple ProRes Codecs