I am creating a custom Tweet button with my own styles however it seems when you opt for customising your own you cannot use the data attributes (data-text, data-url etc.). The utilisation of the data attributes only seem available when you use the Twitter styled button that uses the widget javascript (http://platform.twitter.com/widgets.js).
Here is some code taken from the Twitter dev website. Below is the Tweet button using the data attributes:
<script src="http://platform.twitter.com/widgets.js" type="text/javascript"></script>
<div>
<a href="http://twitter.com/share" class="twitter-share-button"
data-url="http://dev.twitter.com/pages/tweet_button"
data-via="twitterapi"
data-text="Checking out this page about Tweet Buttons"
data-related="anywhere:The Javascript API"
data-count="vertical">Tweet</a>
</div>
Now below here is the 'Build your Own' Tweet button:
<style type="text/css" media="screen">
#custom-tweet-button a {
display: block;
padding: 2px 5px 2px 20px;
background: url('http://a4.twimg.com/images/favicon.gif') left center no-repeat;
border: 1px solid #ccc;
}
</style>
<div id="custom-tweet-button">
<a href="http://twitter.com/share?url=http%3A%2F%2Fdev.twitter.com%2Fpages%2Ftweet-button" target="_blank">Tweet</a>
</div>
Hopefully that all makes sense...