@kickedout the 21px is not really important, it's just to even up all the iframes as alt says - but what I think you were asking is about the !important - this is used to override the inline style produced by the script, however if you're not worried about a 1px difference which is hardly noticeable, just omit that.
I tried to have a look at generated Google+ button code and couldn't get it to work, however I think it actually does produce its own wrapper div, so after me saying it's the iframes that need floated if you have a look at the generated code for the page it may be that there is a div instead for the + button that needs floated too
In that code I can see that the container div is display: inline-block but that won't be working in IE7 and below, and if you do want to float that div you'll have to use !important in order to override the inline float: none; - So, very possibly easier, if you want to go with inline-block and vertically align your links/images to match the iframes.. try this:
I am seeing a new line after the facebook button, but I'm seeing that in all browsers because of the #plus div - the code above makes everything including the #plus div itself display as an inline block , display: inline !important rule is a hack to make IE7 play nice with inline-block too, and because the Google+ button uses it it may be better this way than using/overriding float rules
If you still have a new line in IE8 is the container that these links/buttons are in, wide enough to hold them all?