I've been thinking of getting a motorola atrix and was wondering if it ever got an official update to allow sideloading apps. Did a search and really couldn't find anything.
I had to Sideload a mission critical application last week on my Atrix. Wound up talking to AT&T Business Technical support. They confirmed the next maintenance release for Atrix will have Sideloading just like AT&T HTC and Samsung devises do now. They would not commit to when it would be generally available though. If Motorola is going to have Android 2.3.3 or 2.3.4 as the next maint. level they may be working on a lot of updates for this release. So lets hope it gets finished by the end of the month.
In the mean time I downloaded and successfully used Android Central "Sideloader Wonder Machine"
There is a whole Forum dedicated to this little application, so you should have plenty of support and it is free. Just make sure you follow the prerequisites and instructions and you'll be fine.
I would go with running gingerbreak followed by gladenabled to get side loading. That is what I did the first weekend I had my phone.
Also among the early things I did once I was rooted was things like wireless teathering and what not.
Before I rooted I used Android injector without any problem to sideload. But as suggested here root it. You will get more out of rooting than just sideloading apps.