I got that very same problem! And isn't there a way to disable that base-feature... sucks anyway :D

This may help get you started, but isn't a total solution.

I use the following to achieve a Page Top link regardless of what page or URL I'm on. It may give you something to work with

<?php
$url = sprintf("%s%s%s","http://",$_SERVER['HTTP_HOST'],$_SERVER['REQUEST_URI']);
?>
&middot; <a href="<?php echo htmlspecialchars($url); ?>#top" title="Jump to Page Top">Top</a>

Wrote a little PHP hack which seems to be a little bit more reliable:

First, insert these two string functions at the top of the template "_main.php":

<?php
function get_link_href ($s_content)
{
# This strict function extracts the hyper reference out of a _conform_ <a> tag (MUST contain href="..." !!!)
$current_string = $s_content;
$a_begin_tag = strpos($current_string,'<a ');

$a_end_tag = strpos($current_string,'>',$a_begin_tag);

$a_content_string = substr($current_string,$a_begin_tag+2,$a_end_tag-$a_begin_tag-2);
$a_content_begin_href = strpos($a_content_string,' href="');

$a_content_end_href = strpos($a_content_string,'"',$a_content_begin_href+7);

$a_content_href_content = substr($a_content_string,$a_content_begin_href+7,$a_content_end_href-$a_content_begin_href-7);

return $a_content_href_content;
}

function get_anchorable_content ($s_content,$s_url)
{
# This function converts relative anchor links within $s_content to absolute links (anchor links using the permalink url $s_url)
$final_string = '';

	for($offset=0;;)
	{
	$current_string = substr($s_content,$offset);
	$a_begin_tag = strpos($current_string,'<a ');

		if(!is_int($a_begin_tag))
		{
		$final_string .= $current_string;
		break;
		}

	$a_end_tag = strpos($current_string,'>',$a_begin_tag);

		if(!is_int($a_end_tag))
		{
		$final_string .= $current_string;
		break;
		}

	$a_content_string = substr($current_string,$a_begin_tag+2,$a_end_tag-$a_begin_tag-2);

	$a_content_begin_href = strpos($a_content_string,' href="');

		if(!is_int($a_content_begin_href))
		{
		$final_string .= substr($current_string,0,$a_end_tag+1);
		$offset += ($a_end_tag+1);
		continue;
		}

	$a_content_end_href = strpos($a_content_string,'"',$a_content_begin_href+7);

		if(!is_int($a_content_end_href))
		{
		$final_string .= substr($current_string,0,$a_end_tag+1);
		$offset += ($a_end_tag+1);
		continue;
		}

	$a_content_href_content = substr($a_content_string,$a_content_begin_href+7,$a_content_end_href-$a_content_begin_href-7);

		if(strlen($a_content_href_content) === 0 || substr($a_content_href_content,0,1) != '#')
		{
		$final_string .= substr($current_string,0,$a_end_tag+1);
		$offset += ($a_end_tag+1);
		continue;
		}

	// # ist erstes adresszeichen;
	$final_string .= substr($current_string,0,$a_begin_tag+2).substr($a_content_string,0,$a_content_begin_href+7).$s_url.'&amp;'.$a_content_href_content.substr($a_content_string,$a_content_begin_href+7+strlen($a_content_href_content)).'>';
	$offset += ($a_end_tag+1);
	}

return $final_string;
}
?>

Then, we use the output buffering to get any kind of html permalink. This must be inserted into the WHILE loop which deals with the posts:

<?php
ob_start();
$Item->permanent_link('whatever');
$current_html_permalink = ob_get_contents();
ob_end_clean();
$current_permalink_url = get_link_href ($current_html_permalink);

ob_start();
$Item->content();
$current_content = ob_get_contents();
ob_end_clean();
?>

$current_html_permalink is the permalink url of the post currently concerned, $current_content is its content.

Now we use our anchor-conversion function from above to generate content which is friendly towards the anchors despite the <base> tag in the header:

<?php
echo get_anchorable_content($current_content,$current_permalink_url);
?>

For me, it works just fine. :)
I'm a newbie concerning b2evo, that's why I made use of this rather complicated output-buffering stuff... ;)
Have fun :D:D

Ah, thanks John for the tip. That works!
imjustaguest, I think your php code would have problems then the blog page is displayed instead of the post (instead of jumping to the current page, it will just to the anchor in the page of the post). But that's just upon my inspection, I could be wrong.

You're absolutely right.

I use the hack in cooperation with an introduction-generating script which strips off all html tags, that's why there aren't any anchors displayed on the blog page in my blog.

You may use an if construction which checks upon the existence of the "more=1" parameter to find out whether the blog page is displayed or not.

However, if "more=1" is missing, you may change the $current_permalink_url to the blog page url... ;) (don't ask me if there's a function for it - use some dirty string conversion instead .... :D:D:D)

<?php
echo '<a href="'.regenerate_url().'"#wherever" title="go wherever">wherever</a>';
?>

;)

¥