Spanner, ever consider adding the ability of the sitemap to display dynamic priority and frequency?
When a sitemap displays the same priority and frequency for every topic, search engines lose trust. New topics probably aren't indexed as fast, and most popular topics aren't aren't indexed as often as they could be.
Here's a snippet of vbseo's sitemap:
$prior = 1;
$rate = $threadrow['votenum'] ? $threadrow['votetotal']/$threadrow['votenum'] : 0;
$relp1 = vbseo_math_avg_weight($threadrow['views'], 0, $st['maxv'], $st['avgv']);
$relp2 = vbseo_math_avg_weight($threadrow['replycount'], 0, $st['maxre'], $st['avgre']);
$relp3 = $rate/5;
$relp4 = $max_ping?$mp_array[$threadrow['threadid']]/$max_ping:0;
$relp = $relp1*0.45 + $relp2*0.25 + $relp3*0.15 + $relp4*0.15;
$prior = vbseo_sm_priority($vboptions['vbseo_sm_priority_rt'], $relp);
$dpassed = (time() - $threadrow['lastpost'])/86400;
if($dpassed<3)$freq = 'daily';
else if($dpassed<10)$freq = 'weekly';
else if($dpassed<100)$freq = 'monthly';
else $freq = 'yearly';
$freq = $vboptions['vbseo_sm_freq_t'];
To calculate priority they use an algorithm comparing a value, to it's maximum and average. For example the number of views a topic has received, versus the maximum views and average views of all topics. Then assign a percentage of importance. Here, topic views receive the most priority (45%), then number of replies (25%), topic rating (15%), and finally topicID (15%). I assume topicID is weighted to value new topics higher. Pinned topics receive maximum priority (1.0).
Frequency is determined by last post date; <3 days = daily, <10 days = weekly, <100 days = monthly, >100 days = yearly.