It's possible. Either use the th tag and it'll be a highlighted box like the one if the middle of yours post and the text bold, or simply use the center tags inside the table tags.
Example 1 - with bold letters and highlighted table.
|THIS TEXT APPEARS IN BOLD!|
|This text doesn't.... as you can see.|
|[tableb][tr][th]THIS TEXT APPEARS IN BOLD![/th][/tr]|
[tr][td]This text doesn't.... as you can see.[/td][/tr][/tableb]
See? I didn't use any center tags.
Example 2 - Text centered in the table, without highlighted table and bold text.
|THIS TEXT APPEARS NORMAL|
|This text also, but it isn't centered...|
|[tableb][tr][td][center][THIS TEXT APPEARS NORMAL[/center][/td][/tr]|
[tr][td]This text also, but it isn't centered...[/td][/tr][/tableb]
Look, there I used center tags inside
You've got to know that the table is as width as the longest text in it.
In height, you can only press Enter about two or three times until you get in the middle. That's the only trick I know.
Hope this helps.