Forcing a particular style on searchers / spiders

[roddy]

I'm currently getting hammered for bandwidth - it's all good because it's necessary spider indexing and a subsequent increase in traffic, but I'd like to try and get the numbers down. I've done as much as I can in terms of removing sigs / avatars from guest views. I want to do more. I also suspect this will increase rankings by removing random bloat, a la the archive.

I have Zackery's very nice VB3 Lite style installed - great for PDA use and very light on bandwidth, as it should be.

So what I want to do is place a conditional like this somewhere:

If THIS_SCRIPT = showthread AND (user is guest OR user is a spider)
THEN styleid = 3

What I'm not yet sure about is:
Where do I place this so that it sets the style and doesn't get overridden later? SHOWTHREAD would be the logical place it seems to me, but is that too late? Globals.php?

I haven't been able to identify a conditional along the lines of "is_a_spider", but I see the who's online display identifies Googlebot et al, so I think it must be possible. Or do I just need to list a bunch of useragents?

Also, this needs to be temporary - if a member visits a showthread page, and then logs in, his style needs to be back to default.

I think that's clear - I understand it at least. I'm really just looking for guidance on the above - writing the actual code I can handle, with a bit of trial and error. Not sure I'm doing things the best way though, so comments welcome.