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I believe this could be happening from any number of things. Most likely in your style.css. First you could try adding valign="top" to your block table. But if withing your style.css the table is defined differently then your addition to the block probably will have no effect. Also it could be in the div.blockcontent of the particular column you are placing the block, such as a padding issue or cellspacing issue.
Padding for something like this I have found to be the culprit in several cases.
I used the code you provided and created two blocks. Test Block and Test Block 2 and placed them in the centercenter top colomn. I am not real sure what you mean by HUGE gap because I really didn't have one (using the default theme). However, I was able to reduce what gap I had by editing the theme_blockcenter_c.html of the default theme and the style.css.
theme_blockcenter_c.html- change padding from 5px to 1px
style.css- in the centerCcolumn blockcontent I removed the borders by change 1px to 0px.
You can see the results here:
Your Blocks HereI hope this gives you a direction to look in.
Quest