I don't consider sig campaigns to be spam, although as a personal opinion I find the ones with huge fonts, bright colors, etc. to be somewhat more annoying and create a more negative impression of the advertiser. Simple informative text about something that might interest me is more likely to be received by me as useful information on which I might act.
I remember back when the forum had a different URL and was clutterfusted with annoying signature images and GIF's. I don't mind the text signature ads today but I did find out about nastyfans through a signature link. I say go for it!